try not to worry it might just be whats normal for him. especially if he is active and running round all the time. my 16mth old is a fair bit heavier at 12kg but he has been that weight since just after he turned one he sort of hit a stand still too..
you can try things like rice, pasta and bread if ud like him to put on weight. cheese and avocado are good too.. does he like yoghurt or drink milk coz theyre good for filling him up too.
all in all though i dont think you have much to be worried about, some kids are just naturally on the lighter side and if hes happy and healthy im sure all is fine =)