Oh, poor baby!
I will say a prayer for him. I wonder if he tried using a syringe full of canned food (can't think of the canned food sold at vets especially used for very sick animals that is loaded with calories). I had to syringe feed my very sick cat when she wouldn't eat. Of course, if it's an ulcer, it may not be good for him. Would bloodwork be too expensive for your BIL to have done for him? I hope he will consider a rescue before the other alternative. It could be something very curable and what a shame that would be.
Good luck.