Losing ability to.. shoot straight. Is incontinence far away?

Disposable briefs should not cause problems with his skin as a matter of fact staying in wet/damp or soiled briefs will cause more skin problems.
He should be changing his underwear if they become soiled, same with a disposable brief.
He may not be able to see the bowl of the toilet easily. It is white, the seat is white so I can understand how an aim can go off a bit.
Suggestion...Turn the water off to the toilet, flush so all the water goes out and then dry the bowl. Get some bright color nail polish or even a Sharpie Marker and put an "X" or a "O" on the inside of the bowl. Wait for it to dry then turn the water back on and let the toilet fill. This will give him a spot to aim for.
The mark will eventually come off when it is no longer needed it can be removed. For the longest time I had 2 "X"'s on the bathroom floor so my Husband would know where to stand when I had to change him. Worked like a charm and now that he is gone..the marks have faded and I sort of miss them.

Make him sit.

My DH lived to be 96 yrs 8 mos and the last year or so he too had trouble. Try getting Dad to use a urinal. My DH felt so much better when I suggested it to him and I didn't mind cleaning it. He had one in the bedroom, one in the bathroom and the last year we kept one in the living-room too. If the need arises, he can always ask (or you can ask) the "offended" person to please leave the room. Be aware, the day could come (like with my DH) when I had to support him and hold the urinal for him - but he didn't want to wear adult briefs and as long as I could find a viable solution for us, I never forced it.

There is a product on the market to clean a urinal, but I had Hibiclens for his foot anyway and discovered that just the least bit in the urinal not only cleaned it but the smell was gone. Hibiclens is Pre-Surgical Scrub (soap) and even though DH is now gone, I still have it and use it to wash my hands after touching raw meats.

RE: loose stools: if Dad is on Stool-Softener, stop it. That was what happened to my DH. Also, keep a bottle of Anti-Diarrhea pills handy as diarrhea will cause dehydration.

Get Barrier Cream for his privates - Peri-Guard is my favorite for me - it helps to prevent "diaper rash" - and if he gets a pressure sore (DH had one in his crack), I found Colloidal Silver Gel to be the only thing to heal it. Personally, I preferred the Miracle Silver brand. It will also take down a blister in about 24 hours or less. I don't know why it works on blisters, but I've been thankful : )

You're a good daughter! Bless You!!

Gloria, I know how difficult this can be. My husband and I went through it. He ruined the bathroom floors around the toilet with his lack of aim and our bathrooms constantly reeked of urine no matter how much I scrubbed. He had bowel “incidents” twice, one at his place of employment that almost caused him to lose his job because he simply left the bathroom and the awful mess behind and walked out. Another was in our favorite restaurant. I knew nothing about what happened until the manager came up to us on our next visit and told us that he knew my husband had left an awful mess in the bathroom. He said he had been told by the owner to ask us to leave and not return. I knew my husband would be emasculated and embarrassed, but at that point I had to insist he begin wearing adult incontinence briefs. You may need to insist that your father do the same. Others suggest removing all other underwear from his drawers and replace them with the disposable briefs. He’s never had any skin irritations from them, but hygiene is important. And consider that since he will need help changing, you may want to ask a male relative to step in.

Gloria, have you ever tried a commode?  If he can direct the stream directly into the bucket, it will channel the stream down into the toilet.

You're absolutely correct. This is a delicate subject. In my opinion, it seems that a visit with his primary care physician is in order. Prior to the visit, I would contact said physician to discuss the issue so that the visit is productive. Depends are an option, however, getting him to agree and accept may prove to be difficult. A visit to a urologist also could be helpful.. Unfortunately as Alzheimer's disease progresses, incontinence could occur, but I would pursue the anatomical issues prior to blaming this on cognitive decline.

There are disposable briefs made to resemble men’s underwear. My husband used those (Tena)until he became bedridden. They are pull-ups. I still refer to his diapers as underwear or briefs. It’s important to get his skin very clean to avoid a rash. A barrier cream also helps. I’ve tried many different kinds on my husband and found ones containing dimethicone work best for his skin. The hardest thing I’ve had to overcome was learning how to deal with my husband’s incontinence. I had a couple of meltdowns before I accepted that I was going to have to be responsible for this. He has dementia that over time caused him to become more and more dependent. It’s a kind of death by a thousand cuts as someone once said.

Thank you for your suggestions, I appreciate it!

If the stream is not coming straight out and instead is bending to the side,he should be checked by a urologist.
If the stream is straight out and his aim is truly off, then disregard this message.

Why don't you get him to do what my husband dies. When he has to pee which now is not often, he sits on the toilet like I do and does not stand in front and aim his now very weak stream. He pees like a woman.