I've had dental work on a lot of my cats.Removed teeth that are very painful.My vet told me that it is 100 times more painful than humans cavitys.
My KiKi died of Cancer of the jaw at 16 years old but that shouldn't be the problem.It's not that common.
He probably has Stomatitis or an absess or cavitys or even a fractured tooth.And needs teeth removed.There are many healthy wet catfoods out there to choose from.I give mine both wet and dry.Wet is better because of Urinary Trac problems from it,but I get a very good quality and hope for the best.Good Luck.