Well, our Harry's 18lbs at just 2 years old, and yet when we shower him he looks like one of those adverts for "Please send £5 to help these poor animals in Third World Countries".....
Your Littlie will probably start filling out now, they do tend to once they've stopped 'growing'. Or maybe that's just the way he'll be - were his parents very slim? As long as he's been checked by your Vet, is happy and active and eating ok, I wouldn't worry
Sally + Harry x