We have the same problem with Cade at 6 he weighs only about 38 lbs. He is wirey and busy.Now, Cade is a picky eater, so we try this and that on him, but he is in good shape according to his doctor..We've just decided to make sure he has fruit, veggies, meats, some carbs, available at all times. He is learning to enjoy veggies more and more, and will light up when he knows we are grilling meat or chicken and especially steak..On the other hand there is his brother Carson who the doctor had on iron for over a year, who continually tells my daughter that the boy is "small" for his age...
I finally went with them the last visit, when he began this all over again and I could see my daughter just tighten up, I stood up and asked, with grand parents my size and five eleven like his grandpa, his greatgrandpa was five seven, his dad five 9 just how big and tall do you expect this child to get???
I guess he could tell by my face I was not so happy..He finally admited that Carson wasn't quite so little now!!! YUP and gosh darn..
We have to remember that we are what our family is, 7 foot tall, if my 18 year old was five 2 I think I'd be more than worried..
Your son is a busy boy, this as others have said will change as he grows older..If he is active and not lethargic, eats well as you said..I'd let him be. I'd just keep things he enjoys handy and ready for him..I find that the more of a "deal" we make to the child about eating, the more they turn up their noses..I know, my grandmother turned me off foods in general with her constant following me around with a spoon and bad mouthing my mom about how skinny I was...
Mom, you love your boy and are doing all a mom can do..Love him, keep those healthy meals and snacks coming and he will be just fine.
kadesma