I found a garter snake forum and the good people there gave me tons of advice!

I had an early day at work today, wasn't feeling well and we had enough people so I went home. Made a quick pit stop in the park and filled a tupperware with dirt and leaves, checked under a couple rocks and logs and came home with about a dozen tiny snails, a large slug and an assortment of earthworms.

Offered him a tiny worm with the tweezers and he took it! Now he's looking for more but I don't want to give him another until tomorrow maybe.