Check to make sure the temps are still right, try offering small frogs (I beleive garter snakes normally eat frogs in the wild), earthworms, or red wigglers. He may be still in mating season, though I beleive garter snakes would have mated months ago now. Also, has anything changed in his tank since he stopped eating? Stress could be part of it. Have you been monitoring his weight? Snakes can lose alot of weight without any noticeable signs of weight loss. A vet check may be necessary, especially if you can get a recent stool sample (which is hard to get, since he hasn't eaten in months).