Awww well I'm sure he'll do fine given time, good care and a proper feeding routine. Just remember take it slow. Don't try to shove too much weight on him too fast or handle too much right now. This is just a baby and one that sounds like it didn't receive the best of care so these things take a little time. If you've any concern that this is more than an underfed situation, do consider having a fecal float done by a vet. It isn't an expensive thing, you don't have to take the snake in (just the poop) and it can rule out any problems with internal parasites.