If he's acting normal and eating, I don't think you have much to worry about. How often are you feeding him?

I also have an undersized normal that I think was not fed enough before getting shipped out (66 grams at 2 months old). After getting his husbandry right and realizing he is not willing to take rats yet, he's been eating fine and gaining weight (still on mice). Next step is to try and convert him to rats.