E.K.
Quesadillas and plain meat like chicken breast and meatballs/hamburgers. You might try mixing pureed fruit (applesauce, etc) in the yogurt. Milk can be a back-up protein and diluted juice for Vitamin C, etc. (just make sure you're not giving him more than the recommended serving of either for his age).
I know your son's diet seems limited, but it's more balanced than you give yourself credit for. As I'm sure you know already, kids go through phases. You can't force your son to eat, but you can continue to give him the opportunity to eat new foods. More importantly though, he needs to see you or Dad or Brother eating the same food at least at dinner. At lunch, you might eat different meals, but then share an apple together for dessert. Kids want to do what the other family members are doing, they want to be included. So, even if you're only putting a smidgen of the same food as your son on your plate, seeing you eat it will make a big difference even if it takes a while and even if he only eats one bite of a new food. My daughter will at least try a food sometimes if she sees my son eating it. And, while my son is still picky about certain things, he is definitely in a phase of being very good about trying new food and eating previously un-liked foods(he wouldn't let anything green be on his plate for a very long time). Our kids have some of each food that is being served on their plate and we encourage (we don't enforce it every time) them to at least take a bite, even if they don't want to finish it.
My family gave me such a hard time about my son's picky eating, that I finally asked the doctor about it and she said basically three things: It is completely normal for a kid to be a picky eater, sometimes until 6-7 years of age; judge how they're eating over a period of a week, not daily; and keep offering the foods and watch for signs of malnutrition (weight loss, teeth discolored--might be more, I'm not sure). Well-child check-ups are essential, in my opinion, to make sure your child is staying on track. And, you can always bring him by the office just to have him weighed if that would help give you some peace-of-mind. Good luck!