Identifying and dealing with pain.

I have a long term foster cat, Red, that is not a healthy boy. He has chronic diarrhea that is being attributed to inflammatory bowel disease. He also has a pretty severe case of gingivitis/stomatitis which makes him unable to eat anything hard.
I know that his oral problems must be painful and it's obvious that he's also sometimes uncomfortable or in pain when he goes to the bathroom. Sometimes he cries and strains when he poops. But since he can't tell me how he feels I don't know when he's feeling bad or is in pain so I just know when it's obvious.

He's always been a bit of a bully with some of the other cats. He will slowly stalk them, stare at them and when he gets close he will hit them with his front paws. He seems to just target some of the cats that are mellow and not very tough.
He's not always like that. For the most part they all get along pretty well but sometimes he decides to bully some of the other cats. Like an idiot I always assumed that it was just his personality or something he'd learned from years of living on the street. But recently, after reading an article about aggression, it occured to me that his behavior is quite likely due to physical pain. I feel so stupid for not having realized that earlier as I should have.

Anyway, last time we went to the vet I brought the issue up and she prescribed Buprenorphine to Red. It's quite expensive, at $3 a dose, so we were given 10 doses to start with to give him as needed. The problem is that I don't really know when he needs it. How do I know when he's in pain? I also don't know if his IBD causes him pain other than when he goes potty. Does he have stomach aches at other times? Does anyone have any suggestions of how I can know when he's in such pain that he needs medication?

Another thing I worry about is that he may be in pain quite a bit. Maybe a lot more often than I think. When I've given him the Buprenorphine he comes across as more friendly and affectionate and I wonder if that's due to him being painfree for once which would mean that he feels pretty crummy at other times. If that's the case he may need pain meds fairly frequently but at $3 a dose it adds up. Does anyone know of any other ways that I can treat his pain and make him more comfortable? I realize that he has chronic problems and will never be 100% okay but I'm hoping that his life will be as good as it can.

Thanks for reading my long post

So sorry your guy has such an issue to deal with. He is lucky to have such a caring mom. So many cats I have seen can be so stoic. As you described he seems irritable when in pain but it can be hard to tell otherwise. Maybe try to talk to your vet about alternative treatments for the pain. If he doesn't have any suggestions maybe get a second opinion from another vet, maybe one that only sees cats. They can sometimes be more up to date on the treatments specific to cats.

Good luck to you!