it's always best to consult the dr if you're having questions before you take the matter into your own hands...if he's healthy then he'll probably just grow into his body...when he gets older and starts to build muscle he'll bulk up
At 10 yrs. my son is about 41/2' and around 53lbs. His Dr. seems to think that its fine, and often says that he wishes more of his child patients were like him. Like you, I sometimes can't help but worry, but then I think about how he won't likely have weight related problems early in life. My husband is over 6' and 200lbs, but was also a small child. We were told by our Pediatrition that supliments and drinks like ensure, will often shut down hunger in kids as their nutritional needs are met and they are efficient fuel burners.
If your son is eating a balanced diet dont worry and don't try to give him artificial calories or bulk him up.... at 9 he is pre pubescent and has not even started putting on muscle mass really.
Be very careful about making such a young child worried about his weight - it could result in obsession and eating disorders.. He is at an imressionable age.
If he is very active then he is getting enough goodness regardless of weight, if he was weak and tired and lethargic and thin then I would be worried....
Talking to a couple of friends with sons around the age of 9- they are saying between 55-75lbs