I have lots of advice on this one. First, don't panic. Second, don't get sucked in to giving him the sweet, vitamin packed meal substitutes.
My kids were all in the 25% for height and weight and didn't eat much at all when they were little. My goal became to always have healthy food offered to them and then not sweat it too much. Just like dieters, offer 6 small meals a day. If he likes something that is healthy, have a lot of that on hand and build on it. Grapes, raisins, nuts, peanut butter, cheerios, mac and cheese (as healthy as you can afford).
One of my kids hated bread. She ate a flour tortilla with peanut butter and applesauce rolled up every day for a year. I would put ground up spinach and carrots and zucchini in their spaghetti. One of them liked turkey rolled up in a piece of cheese so we went with that. Veggies with salad dressing are good finger food.
Try not to change your diet to accomodate him completely, just incorporate a little of what you know he will eat with a little of the "big people food." So if dinner is meatloaf and green beans and he will only eat mac and cheese, give him a little of each. Even if he doesn't touch the meatloaf, it is there and you can encourage him to give it a try.
Another trick we used was to let them help shop and pick out fruits and veggies and help plan the menu, we also let them grow their own. Amazing how a kid will eat a green bean or pepper if he planted the seed himself. He isn't too young to do that! He is also old enough to make his own food. Give him a butter knife and his favorite ingredients and watch him go to town. He can cut and spread things. Give him a plate with some cheese slices, crackers and a butter knife and let him cut the cheese to go on the crackers. Spencer was making his own PBJ's when he was 2. Bagel or english muffin pizzas that he puts together and watches them cook. Let him pour the measured water into instant oatmeal and put it in the microwave. I found that the more involved they were in making the food, the more inclined they were to eat it.
If it helps....the kids are now 18, 16, and 14 and very healthy. The younger two are taller than me. My son (14) is wearing a size 12 shoe and going through a gallon of milk a day He loves to eat lamb and duck and limas and just about everything else. The 18 yo is a sous chef at the Ritz Carlton and the 16 yo's favorite food is sushi. They really do catch up and if you don't totally cave and have them try other things, eventually, they will like more and more variety.