What about making traditional kids favourites but tailoring the recipes so they are low fat? For example, you could make pizza using sliced english muffins, low fat cheese, and cut up veggies (mushrooms, green/red pepper, onion).
You can also just substitute higher calorie foods for lower calorie versions. Cheese, milk, butter, etc. all come in lighter versions that can save a lot of calories.
Another idea is grilled cheese sandwiches, but baking them instead of pan frying them. Toast some bread, slice some low fat cheese (or use fat free american cheese slices) and put it on the toasted bread. Broil it until the cheese melts. Add on a pickle slice, some ketchup and mustard and you have a really great lower calorie grilled cheese.
To help your son get more veggies in his diet, try making soups that have vegetables in it. Most kids will eat veggies if they are "in" things instead of expecting them to eat a pile of steamed brussels sprouts or something.
Portion control, as you mentioned, is a big thing as well. Your boy can still enjoy his favourite snacks and treats, but just limit them. Perhaps you could put some treats in individual ziplock baggies and let him choose 1 per day. That way he wont feel deprived or like he is on a "diet" but he wont be eating mindless portions.
Oh, one more thing, don't forget about the benifits of good exercise! If you all do something together as a family it wont feel like work. What about going for a nice walk after dinner each day (or even every other day)?
