If you have no plans on breeding...a good month brumation should be good to stimulate appetite. I have garters that either aren't big enough for breeding or don't have a mate for them that go off feed in the fall. I put them down for a month and bring them up. with the temp you are keeping him at, I'd keep a good eye on him...thats a little warmer than I would brumate at. I have found garters out and about occasionally with temps here in the low 60s and upper 50s....not very often but they are definitely more cold tolerant than most snakes. I would brumate a little colder if possible and for a month. after the month, start warming him up. after about a week you can try feeding again and he should start eating. forget the grubs, garters don't eat insects. they eat worms, slugs, fish, amphibians, and rodents.