It sounds like you are going to have to make a tough decision for your little fella. He will be in serious pain at the moment and metacam is only like aspirin. Dental problems in small animals are not like those in people - because their teeth grow continually, once they have problems they tend to come back, and small animals hide pain really well. Once they stop eating there is a serious problem and there is a difference between keeping them going while a problem heals and they will be fine and just keeping them going with syringe feeding.
Have a serios conversation with a vet you trsut about his quality of life and outlook. I know it is hard with a much loved pet, but it is also our responsibility to make a kind decision when the time comes. You haven't done anything wrong trying to treat him - this post isn't meant a criticism of you trying to save the little fella and I also understand why you don't want to hand him over to someone else.
If you want to try treatment, either via Furry Friends, who have a lot more experience than I do, or via another vet ask about pain relief after surgery. There are much stronger painkillers available, which I have used very successfully with chinchillas, but vets are understandably nervous about giving them out.
Whatever you decide It will be because you care for the little fella.
Paula