Not sure how you structure meal and snack times, but it would be a great idea to add two snacks to the 3-meals-a-day if you haven't already. Planning for a snack time at least 2 hours before a regular mealtime may help him not ask for food as often and help fuel his growing body - after all, those little bellies can't hold a lot of food at once! I would guess that if you never gave your son food for a reason other than hunger (feeding him to stop him from crying as an older baby, etc.) than you are probably right about him not eating out of boredom. As long as he is active and maintains a healthy weight, you can feed him all the healthy food he seems to need (within reason, of course!) :)
If you're already doing that, perhaps a filling breakfast would be a good way to start the day. Oatmeal cooked with milk and a small amount of butter, brown sugar and cinnamon or jam would be a healthy start. Eggs are also a high-quality protein that will help fill him and you can even add a little grated cheese and maybe a spoonful of (mild!) salsa. When trying to fill up on healthy foods think of what nutrients fill us up - first is fiber, 2nd is protein, 3rd is fat, 4th is complex carbohydrates (aka whole grains). The oatmeal has great fiber and complex carbs and the eggs have protein - add a little from the other food groups, especially veggies, and you can have a nutritious and tasty meal. The same fiber-protein-fat-carbs rule is true any time of day, so you can certainly apply it at any meal or snack. Also, try serving the 'same old thing' in a new way - think shapes (cookie cutters) or making 'trees' out of broccoli and things like that. Perhaps asking your son help you prepare his food will help him be more adventurous with new foods.
Here are a few other food combos:
apple wedges and PB (try coring it, slicing into rings and making a 'sandwich' - even add raisins on the PB. My kids love this!)
cheese stick and pear slices
tuna or chicken salad (light on the mayo)
hard boiled egg
2% fat yogurt with banana cut up and mixed together
PB and banana sandwiches, always using whole grain breads
These are all things my two kids eat and LOVE - Hopefully that helps! You can always check the American Dietetic Association's website for sound nutrition advice at any stage of life - www.eatright.org. Also, your pediatrician may have brochures about ideas to help young kids eat healthy foods. Stick to the Food Guide pyramid and you'll give your son a great, healthy start!