The most important thing is your Pediatrician concerned?
I have a very petite little girl, she is seven and weighs barely 45 lbs, is 46 1/2 inches tall. At one year of age she was only 16 lbs.
She has managed to finally kick it up to 15% but was well below all her life.
My Pediatrician said if they have good appetites, good energy levels and are growing in height he is not concerned. So I have had to really relax a bit. She will eat a TON during a growth spurts, she is full of energy, rarely gets sick so I just have had to not worry. She is healthy but just able to not gain a lot of weight.
If your Pediatrician is concerned, then he/she should be giving you a nutritionalist to consult with and a game plan to help figure it out. He is growing in height and that is a great sign he is fine. Does he have lot's of energy, able to be attentive in class, does he get sick often?
You shouldn't give him high fat foods either unless your Dr said to, that can cause rapid weight gain or other issues. Bottom line is if he has a high metabolism at some point in his teens it very well could stop and you don't want him having weight issues or self esteem issues.
If you trust your Dr, let him or her guide you on when to be concerned and if they are what you should be doing.
I make sure my daughter eats three healthy meals, I don't add anything additional to her diet, she drinks 1 percent milk like the rest of us, I let her have snacks, make healthy choices for her, allow her the occassional junky food and just chaulk it up to her being fortunate to have a high metabolism. My Dr said during puberty that all changes and if they have fatty eating habits they can gain very fast and that is not good.