You can bake just about any vegetable into a muffin. Shred them when fresh and incorporate them into a yummy muffin recipe. I prefer making whole grain muffins, but you can use what you currently use. Don't let him be in charge at the table. We used to go to Dr.Denmark, who turned 110 on Sat. and still does phone consultations. She would say to make the meal healthy-a small portion of protein, veggies, fruit and starch. Don't allow snacking or juice in between cause he won't be as hungry. Sit down with him, and when you are done eating, clear his food away (unless he is actively still eating). Don't let him eat anything else till morning. He'll soon learn that he needs to eat the food that Mommy caringly makes him. Too many kids that come play at our house live on hot dogs, chicken nuggets and french fries. They have a future of major health issues brewing.
One little trick I did with my kids was that they had to eat their age in bites of something they were not fond of. After a while it no longer became an issue and they are all great eaters now.