If you caught him, and he won't eat after 2 or 3 weeks and he's that small, return him to the wild. Baby garter snakes wither and weaken very very quickly without regular food intake. If it were a "failure to thrive" case, the snake probably would have died during brumation. Those ones never make it very long at all. The stress of captivity may be too much for this one. Not all of them adjust!