Not sure when to tell the family to come?

From what you describe he is not at EOL (End of Life) but he is declining rapidly.
There will be indications when he is "Actively Dying"
The important thing is and this what you may have intended with your question..When to get family there to see him while he is still aware of himself, his family and surroundings.
With the holidays soon here you might want to arrange something around the holidays. But how well does your Dad do with large groups? I know everyone will want to see everyone else but this might be overwhelming. I would keep visits so a few people. And keep the noise down as that can be frightening. You can all gather for a lunch or dinner later. Or leave it to individuals or individual families to make the visit on their own time, but you might want to stress the decline and let them know if they wait to long he may not recognize them.
(By the way the "making up stories" he is not making up a story, he can not find the word that describes where he is, a type of Aphasia maybe)

And since this is me responding....
Please talk to his Doctor about...
Hospice. It is never to early to begin that discussion.
Decide now if you will use a feeding tube...I strongly discourage that.

Also..
Discuss with the AL facility he is in now will they allow the use of equipment to move him (Hoyer lift, Sit-to-Stand) some will not allow it.

And I have to ask..How is your Mom doing with all this? I do hope she has a lot of support as well.

I had a friend diagnosed with advanced dementia in late June . She went into an assisted living, her condition worsened at warp speed. I visited twice a week, sometimes more. At times she really felt she had no one, had no immediate family left. There were neighbors she lived near all her life, no one visited her. A month ago, hospice was brought in, people would ask " how is she?", " we plan to visit", blah, blah, blah. Again, nobody. She fell Nov.5, hospice said she was ok except for the cut and bruising. I was there on the 6 th, clearly she was worse. Nov. 7 , hospice nurse called me, she had passed. So my advice is you never know if they have a tomorrow, visit when you can, my friend lit up every time I was there even if only for a second.

Here's a suggestion for those who live far away, have jobs & family obligations to juggle, for whom travel would be a financial burden, or when the timing might be in question. My cousin lives on the other coast. When she traveled to my state on a business trip several years ago, I went to see her. She recorded a nice long video message on my phone for my dad, just in case. She talked about the ways he had inspired her career choice, other decisions in her life, and fond memories, among other things. It is a warm, loving, upbeat message. I play it for him from time to time and it's a new blessing for him each time. Messages can be sent in several formats so you should be able to assemble a small video "library" from family members. For those who can't make it in time it will be a comfort to them to know that their loved one was able to see and hear them express their love and gratitude for their presence in their lives.

When my mother became truly impaired, I spoke with my children and left it up to them. My young grandchildren did not visit more than once a year and saw her for the last time about 6 months before she passed.

My children (at that time in their early and mid-30’s) wanted to visit Nana; my daughter more than my son. My mother could get really nasty. She also spent 90% of her conversations with me accusing the entire population of the facility of sex orgies. I did not want my kids or especially my grands exposed to that.

Invite them to visit according to their their own schedules and desire to see him. Don’t press them or make them feel guilty for not visiting. My mother was especially nasty to my son on one visit and I told him he was free and clear of visiting her again. Don’t make the visits hours and hours and don’t have everyone pile in at the same time. My mom was stressed out when we all came to see her at Christmas once. She nearly attacked my husband when she thought he was a strange man.

The best you can can do with this disease is play it by ear. There are no hard and fast rules.

I'd suggest researching the 7 stages of Dementia, so you can identify approximately where your Dad falls on the spectrum, and I'd also suggest looking into the signs of impending death so you can tell when his passing is imminent:

Global Deterioration Scale for Assessment of Primary Degenerative Dementia
https://www.fhca.org/members/qi/clinadmin/global.pdf

Technically, the death process starts when the patient can no longer swallow. They either have to be artificially nourished or allowed to pass as painlessly as possible.

Let people know now what is happening. It will give those who live further away the opportunity to plan a trip. With the holiday season approaching, people may have plans to go away and need time to make alternate plans, if they can change plans that include flights and hotels at this late date (many places have a 60 day cancellation policy during Christmas).

Some will want to see him sooner, other may wait until the funeral, but it is important to give people a choice. My Aunt did not come when Granny was dying, she chose to wait and come for the funeral. She knew she could not afford two trips and felt it more important to attend the funeral.

I'm not sure if you are asking family to come to say goodbye while he knows them or before he passes away. I'm no expert, but, I would have all who want to be recognized to come now. Dementia patients can progress rapidly. I recall how startled I was the first time that my LO didn't recognize my mother.....then me....it was tough. So, I'd go while he knows them and can interact. You can visit later, but, he may not know who you are and not be able to really communicate.

Now my LO has lost all verbal skills. I'm so grateful for the visits we had where we talked about the good old days. Even though, her short term memory was gone, she still recalled her teen and childhood years. Then, that's gone. So, I wouldn't delay.

Perhaps you might want to share with them the gist of your comments here.  You could add that no one knows when he will no longer know anyone and that visits at any time are welcomed.

Grace + Peace,

Bob

From my observation as long as he's eating he's fine, physically at least. If I were you I'd just keep them updated with any setbacks and describe his general physical and cognitive condition and leave it to them to decide whether they need to act on it.

The way I did this was to explain to family members that it was their choice whether to come or not and if they wanted to come, there's a few things to consider in their timing. One was that the timing was very unsure but since Mom was growing weaker, the best chance of having a meaningful interaction with her was to come sooner rather than later. It might not be "saying goodbye" but it might be a chance to say (and hear) other thoughts.

The way it broke down is this: Her brother (my uncle) chose to come sooner, for Mom's birthday which was the 4th of July. Mom was well enough to go out for her birthday, which was the last time she did that with anyone. She went into a serious crisis the day after he left and died two weeks later.

Two sisters came early (on both sides of my uncle) and came back again a few weeks later to be with Mom at the end. My brother was planning to come at the end of July but moved his trip up to see Mom while she could still talk to him. One sister didn't come at all and claimed later than she had no idea Mom was so close to the end and nobody let her know. (Totally untrue, but she was unhappy with her choice not to come and needed someone else to blame). I think everyone else was satisfied with the choices they made and it turned out best for Mom as well.