What's going on and what can help?

Fresh air, exercise and company are all appetite stimulants. You have to be careful that he isn't cutting back on liquids because of diarrhea because that can cause UTI.

Thanks fir all the responses. FIL is a Veteran so his VA doc came to see him and she is giving us a referral for hospice. He has lost a lot of weight. I know dementia plays a part in him eating less. My concern is more is coming than he is taking in. His BM is watery, light brown, no foul or fruity orders.

You mention in your profile that James has dementia, specifically Alzheimer's.
Stopping eating is common with dementia. Depending on where he is in his journey with dementia.
Contact his doctor, explain what has been going on. One of the tests that might need to be done is a Stool Sample and he might not even have to go in to get the test done if you can obtain a sample at home.
If it is C-Diff you need to be VERY careful as it can be transmitted. And it can VERY difficult to control.
Again depending on where he is in the dementia the body stops requiring as much food. If he is not real active decreasing food would be normal as he is not expending the calories. And at some point the body will get to the point where the calories needed are to keep major organs going. The brain, heart.
But contact the dr. and let them know about the ongoing diarrhea.
If you can not get him on a scale weigh loss can be monitored by Upper Arm measurement as well as in general how clothing is fitting.
If he is truly done with doctors ask his doctor about Hospice.
Hospice will provide you with supplies and equipment as well as support you need. The goal of Hospice is to make sure he is comfortable.

Is he on any medications? I know some anti-anxiety meds can wreak havoc with the intestines.

Maybe tell FIL that fixing his issue could be as simple as adjusting his medication. He's free to refuse anything beyond that.

If your FIL is done with the "poking or prodding" then it may be a good time to get hospice involved, as they will keep him comfortable just like he wants. They will also try to get his diarrhea under control best they can. The fact that he's not eating much and drinking only 16 oz. a day is concerning for sure. He has to be severely dehydrated, especially with him having diarrhea for the last 2 months.
Depending what state you live in you will either need a doctors referral for hospice, or you can call for an evaluation yourself. Just make sure you do your homework, as all hospice agencies are not created equal. Best wishes.

Sorry to be graphic, is it yellowish &/or fruity odour? C'diff?

Is he losing weight? With diarrhoea and low food intake, you would expect him to, and you need to monitor it. If he really doesn’t want to eat or have any ‘poking or prodding’, he may be ready for palliative care which is just comfort care. He doesn’t sound like hospice is relevant yet. Palliative care could be sold as getting help for you, rather than an assessment of him, and they will come to you. Best wishes, Margaret

I don’t know how you can help him if he won’t agree to go to the doctor. An examination is clearly in order.

My mom once said to me, “I am never hungry.” So, I asked her, “So, why do you eat? Is it because you know that you have to eat to survive?” She said, “Yes, that’s why.” Some people don’t have much of an appetite. My mom didn’t eat complete meals. She was extremely thin. She probably didn’t even weigh 100 pounds at the time of her death. It seems that some people get skinnier as they age.