Really sorry to hear you and piggie have had such a difficult time of things.
It is really important to keep giving syringe feeds to put/keep weight on but also try and tempt him with his favourite vegetables, perhaps parsley, fresh grass and timothy hay.
It sounds like they had to do quite an extensive op because of the overgrowth so he might not have much tooth at the back to chew with - and it's likely to be still quite sore. I know I'm not a rodent, but after I had a tooth pulled, it was weeks before I could even feebly chew on that side.
Has the vet prescribed pain relief, like metacam? Has he been back for a check, just to be sure there isn't a post op infection?
I had one boar who had dental problems due to unequal wearing down of the teeth. He had to see a rodentologist to have them clipped and filed about once a fortnight. If he happened to do a particularly close clip and file, piggie didn't show alot of interest in food for several days and had to have syringe feeds until he perked up. Although he had to "go to the dentist" regularly for the rest of his life, he lived another 2 1/2 years with a good quality of life until he was about 7 or 8 (he'd been a rescue, so they weren't sure how old he was when he was brought in.)
Do you have a rodentologist near you, or can your vet do dental treatment without general anaesthetic? I know for extensive ops, anaesthetic often can't be avoided, but teeth can be maintained by clipping and filing every 2 or 3 weeks without it, so long as the rodentologist or vet has the right equipment.
It's all down to quality of life really. I don't want to give you a false sense of hope that all will be okay as piggies can be very badly affected by stress generally. If you think your vet is cavy savvy, they should be pretty realistic in advising what the prospects for the future are. If not, hopefully someone here can recommend a good vet or rodentologist.
Take good care.