That sounds like a negative attitude to go into treatment with, I would be upset too. Dental problems can be severe, but many cases are not and with some supportive syringe feeding and pain meds, your bunny could do very well! I know there are other people here with more experience than me with syringe feeding (and really, I have no knowledge of bunnies anyhow though I have syringe fed hedgehogs and guinea pigs.) I hope he makes a full recovery and proves the vet wrong.