Chinchillas

I have 2 chinchillas about 4 maybe 5 years old my one chinchilla is having problems with his teeth took him to the vet. The vet put him on antibitic for two weeks he got a little better keep him on the antibitic for another too weeks. He has good days / and bad so days he eats some days he don't I have been babying him and hand feeding him I don't want him too lose any more weight then he has all ready have what can I give him to help him gain weight please help thank you for your time