When you go to feed him try thawing out two hoppers. After he eats the first see if he'll eat the second.

Both my pythons are still slightly thin. Though I do not know their exact age, one of them (my bigger girl, Probably seven months old) has gained a significant amount of girth over the last month. She has moved up to good sized adults. Her diet is amazing. As for my little guy who is probably four to five months old he is still thin as can be and eats adequately.

You best bet is to feed him as much as he wants and just wait it out. I am not positive on this, but I think the girth comes later in their first year.