Is it just me that thinks that his body is on the small side? Comparing his head to his body?
What do you know about how the breeder fed the litter? If they group fed the babies at a bowl it's possible that this little guy got pushed aside by the bigger kids. If he's eaten worms for you that's great, keep feeding him every few days and try to get him eating the chopped pinkies. I'm sure that he'll start putting on a bit more weight now that he's getting more individual attention away from the rest of the litter.