Have you tried offering him plain meat baby food? Just plain baby food with no veggies or anything in it.
Can I ask how long it before you noticed Boo’s mouth wasn’t bothering him anymore? I have a kitty named Sur in a very similar situation as the cat in this thread. I got him a kitten within a week he was opening his mouth very wide after eating sometimes pawing at it clearly in pain. He stopped eating. Sub-Q fluids, labs, anesthesia to examine mouth and throat, onsior, cernia, buprenex, antibiotics, 11 days, and $1000+ dollars he is eating more, acting more like himself, but his mouth still bothers him 50% of the time and he paws at it and starts hacking after eating....If it is calicivirus, something similar happened my Boo six years ago. He caught it from the other two that I adopted from the shelter. He was hiding under the bed (very unlike him) and drooling profusely. His mouth was clamped shut. The vet saw a huge ulcer on his tongue and a special blood test confirmed calicivirus (as well as feline herpes virus). He was in a lot of pain. Since the vet did not want me messing with his mouth, they put him on a Fentanyl patch for pain, which lasts for three days. Wow, what a difference that made. He ate as soon as I brought him home from the vet. I did bring him back one or two more times to get the patch put on again. They did give him some other medications, but I don't remember what they were. Onsior might have been one of them....for both pain and inflammation. I do have him on l-lysine also. There is a debate as to whether l-lysine helps with calicivirus, but he hasn't had a flare since. ::knock on wood::
I hope your boy recovers soon. Sending lots of healing vibes!