REALLY foul smelling, like drive you out of the house type.

I had a friend who went through much the same stuff. He was finally diagnosed with Whipple's Disease - not something they often test for because it is kind of rare. Just throwing it out there. I hope that it resolves.

Adios,

Pizza Bob

Not offering any medical advice whatsoever. Do your own research. I had very similar symptoms some years ago. I never discontinued traditional medical advice and medications. But a friend suggested that I add mastic gum to the mix and see what results I got. "Mastic gum" is not an actual gum. It comes in a pill form. Here is a link with a lot of information. I got great results with the product. No current symptoms at all.

https://www.iherb.com/blog/mastic-gum/1346

“78 yo male who smokes (since the 60s) drinks too much, and doesn’t follow doctor advise.“

Don’t make this too complicated.
I’d start with the basics first. The simple explanation is often the right one.

Having just gone through something very similar, check his pancreas.

Often overlooked, your dad has several classic symptoms.

Surgery was called for in my case to remove the portion of my pancreas that was inflamed.

Alcohol is often the source of issues here. In my case it wasn't, but typically it plays a large role.