:Your garter probably wont eat crickets, and in facts, the crix can hurt him if they're big enough. As for feeding him, try earthworm that you will hold in front of him with small pincer, or put some fish piece in a dish. My garter like to eat his fish piece in his water bowl. Is you garter big ? whats his size ? The sphagnum moss might be okay but for my part i put my snake on paper, easyer to clean :)
He's pretty small. Probably about 40 cm long. To prevent crickets from biting him, I put some veggies in the tank for them to munch on instead of him.