Hiya Joy although everyone has pretty much filled you in on the basics of keeping that cute little garter snake, I figured it wouldn't hurt to point out one other thing, regarding those bait-worms:
Make sure if possible that they are not the sort of worms that come from commercial compost depots, or otherwise gigantic compost heaps. Apparently these worms can be too acidic for garter snakes, and though they would probably eat them, they will make snakie sick.
Personally, I don't know how to identify these 'red-stripe' compost worms, because I have never seen them, but it might be something you should inquire of the seller.
Your son strikes me as a very inquisitive and bright kid and it's wonderful that he has such an interest in the natural world. Just great!
For the record, my snakie loves worms, salamanders, newts, and pinky or fuzzy mice. She does not seem to care for fish in any shape or form, fresh or frozen or live. But, garter snakes have been known to adjust their dietary preferences during their lifetimes, so who knows, your sons snake may one day like something it didn't before, or vice-versa.
Anyhow, welcome to the forum, to you, your son, and the very pretty garter snake you now have -- have fun!