Cat is having mouth issues

My cat has a gum issue and his mom had it before she may have passed it on to him. He has been fine up until this point and im not 100% sure what to do. Hes been in alot of pain and I cant afford a vet visit. What should I do? His teeth are pretty bad and he has moments where hes jumping and howling because of the pain. We have been giving him watered down wet food to help with eating.