Is he eating? If he is, then I wouldn't worry too much. I have two young marginated tortoises that are the same age, but one is twice the size of the other. Despite this size discrepency, the two are healthy as can be.

Some young sulcatas take time to become the dominant animals they can be, and some are certainly more dominant than others. What counts is whether or not the animal is eating. If not, you may have a problem.

T.G.