Offer live, a hopper mouse or rat pink/fuzzy. Your snake may refuse, but if it has been 3 weeks, he may not. Babies can't go as long as adults, and if your description of size is accurate, it sounds thin. I have picked up babies at reptile shows who hadn't eaten, but after settling in for just a few days in a nice safe place ate for me no problem. If he doens't eat, offer again in 4 or 5 days. Babies need to eat every 5 days or so anyway. Good Luck!