I pretty much would only eat chewy bread with pesto and swiss cheese for school lunches at that age.
I say brainstorm with him about what he wants and what you know is possible. Maybe have a paper with different categories that he can help list and choose from. Take him shopping.

I had a picky lunch eater, the hardest part being that she wouldn't eat sandwiches -- she said they got soggy. Even if she was right, she still needed lunch. With that and other won't-eat's, for a long time lunch was basically a small yogurt, peanuts in the shell and a piece of fruit. The school called me about her lunch, probably because they didn't see a bologna on white bread sandwich, chips, cookies... She was healthy, and the meal had plenty nutritionally. Anyway, moral is, keep it to good food and let your son explore healthy choices with you.

There is a great new lunch box I saw recently that keeps all things separated. http://www.shopkirdevries.com/kotobuki-2--deck-frog-bento-bo2.html I don't have a picky eater, but I have a "must keep things seperate" eater. So it's perfect and might help you.