I wouldn't worry just yet. Let him get used to the enclosure and used to being around people. From what I've heard, garters can take up to a month to get used to a new habitat, and can go extended periods of time without food. Heck, my Little Dude (a female sirtalis) has barely eaten the past few weeks, though prior to that, she was gobbling up tilapia like it was going out of style.

One thing you could try might be to let him have some privacy while eating. If a snake feels threatened, they won't eat. But if you just put a nightcrawler or two in a bowl and leave them there for a while, he might grab one for a snack.

Point is, as long as he's healthy, he'll eat when he's ready. No need to rush things or worry about it. Offer him the food maybe once every two or three days, but let him determine when it's time.