I feel sorry for the little guy. I have been having problems with stones for several weeks. It is really ugly. But I guess not so painful for him.
Without knowing what kind of stones they are, if it is stones, they really can't accurately tell you what to avoid feeding him. It might be wise to cut back on high oxalate vegetables, although I have not personally.
The most common stones are calcium. But getting enough calcium from food seems to help prevent stones. (On the other hand, calcium supplements seem to increase the risk of stones.)
I have been hoping that vitamins D and K, and magnesium would help prevent calcium stones. However, I don't know what kind of stones they are. I am supposed to get a fine mesh strainer to try to catch one. But I haven't gotten around to it.
Give him lots of fluids, of course. You already knew that. I drink water with lemon or lime.
Good luck. I hope you can beat this soon.