Growth works in odd ways. We don't know that much about it but we do know that it tends to come in spurts. From my own observation, kids sometimes resemble little bears, storing up weight till the growth is ready to catch up. But this usually happens more in the adolescent years.
The goal with children is to teach good food habits by example. The goal in weight management with children is not necessarily to make them lose weight but to maintain their weight where it is now and wait for the height to catch up during a growth spurt.
Many people are not aware that food behavior is learned and is not instinctive. It's often an expression of personality and choice. Parents should not get upset when a child doesn't want a particular food. Within the framework of good nutrition, everyone should be allowed to choose what they want.
If your son drinks low fat milk, eats eggs, cheese and peanut butter, he's probably getting enough protein. The recommendation is 42 grams of protein a day which could consist of one or two eggs, two tablespoons of peanut butter, two glasses of milk, and one or two slices of cheese. Remember that breads and other starches contribute proteins as well.
INVOLVE YOUR CHILD
We never want to force a child to eat but it's okay to say to him, "Listen, I'm afraid you're not going to grow properly if you don't eat a well-balanced diet. I see you like pizza, bread and pasta and we can have that but you really need to eat fruits and vegetables too." Sit down with him and see what kinds of fruits and vegetables he likes and how he likes them prepared. He should be eating four servings of vegetables and three servings of fruit daily. That's not as hard as it sounds. Remember, a glass of fresh-squeezed orange juice is two or three servings of fruit right there.
Together with him, make a plan. Don't be too rigid about the amounts he needs to eat. But do limit the starches. When you're serving pizza, spaghetti and other high starch food he likes, two servings a meal is probably enough. Tell him if he's still hungry, he can have a vegetable or fruit.
Involving a child in food preparation sometimes helps. He's old enough to peel a cucumber or carrot, or slice fruit and arrange it nicely on a plate. If you have a garden, have him plant some vegetables. Take him shopping and ask what fruits and vegetables we should try. Try taking him to a farmer's market -- that's a wonderful experience at any age.
Encourage your son to do more sports and physical activity such as riding a bike with an older sibling or his parents and/or take him out to the basketball court and throw a ball around for half an hour. Pick things that he and you both enjoy. This should be fun, not work.