I would just keep an eye on him. I will tell you a little story.
My baby sister (she's almost 11 years younger) was skinny and tall all of her childhood. I bean like #2 pencil skinny as was I. After she graduated high school and went off to college she like went nuts on all the stuff that my mom wouldn't let her eat. I mean nuts on this stuff, sodas, cookies, chips. My sister then started getting larger and larger and now at age 27 she is 240 pounds, can't breath well, blood sugar is messed up and feels horrible all of the time, her self esteem is in the toilet and she's with this guy who is larger than her now and is a slug, won't work longer than a couple of months at a time. All because she thinks she can't do any better because she's so heavy.
So, while keeping your kids on a healthy diet is great. Keeping things from them that other kids eat completely isn't a good thing in the long run.
The doctor should show you on a chart what percentile your child's weight and height is in relationship to his age and those of kids his age. So if he's in the 50th percentile or close to it then he's fine.
Just keep him active and stay away from the sugars as much as possible.