We were taught that what was important was that both the height and weight were remaining fairly consistent. If the height, for example, increased to the 90th percentile from the 80th percentile, and the weight increased from the 40th to the 50th, they're both on the same basic track. But if the height increased only from the 60th to the 62nd, but the weight increased from the 50th to the 95th, you have a problem. Or if the height increases from the 50th to the 95th, and the weight goes down, you may have a problem.
At age 11, when your son participates in family dinners and eats a variety of foods, it sounds like you're doing just great. Food is fuel, not a reward. If your son has enough energy to participate in life, and is happy and active, then he's getting fuel. Just try to serve real foods, recognizable vegetables (steamed or roasted), and try making your own homemade taco seasonings and limiting processed foods. Salads are great and can include sliced steak or tuna or chicken. Some days, serve fruit for dessert (in its original form, not in sauce). It's amazing what happens when you place a bowl of berries, or fresh pineapple chunks, or grapes, on the table.
And don't let your son know that there's this discussion going on with this out-of-date doctor about food. Don't push your son, don't fuss over food. Make the meals relaxed and it sounds like he'll have a good attitude about food.