It's so hard to know what to do (or not to do) to help... if you can get him to the vet, that would be best, IMO. But all things being as they are, sometimes that's just impossible for any number of reasons. Keep tempting him. If he likes tuna, then maybe a little mackerel or salmon might be tempting. (As long as Patches doesn't eat it all, of course!).
I've got one I'm doing a death watch on right now. But she's just over 20 years old... and she looks it. Still has a decent appetite, and I'm letting her sleep on my pillow and anywhere else on the bed she wants. All she wants is to be near me. Funny thing is, this cat was always my hubby's cat... never really liked me. Suddenly, we might as well me conjoined twins!
Tiger may just have been looking for a safe, welcoming space for his last months on earth. To have picked you was an honor, if so. That came out clumsy, but I think you know what I mean. I had a cat like that once. Came for Thanksgiving dinner, lived with us for a few months, and then died in his sleep. Loved the hell out of that cat while he was with us. Still miss him.
Holding you all in my heart; my Bast bless you and watch over Tiger...