Start with ingredients he likes and go from there? Can you give us a list? Meat/fish/protein sources, vegetables, carb sources and then flavours/sauces.
Pasta bake - with any meat/veg he likes plus a sauce, cheese if you want or top with breadcrumbs and leave in oven. Can freeze/chill leftovers. OR do in slowcooker without topping (start pasta dry). Again chill leftovers and microwave.
Flavoured/fried rice - cook the rice with a stock cube and then add other ingredients. Fry through in wok or just stir them in. Needs a lot of flavouring but works.
Jacket potato fillings - stews, curries, baked beans mixed with various things, you can freeze these.
Mild curry you can just use meat and onions or include chunky veg he can pick out. To be batch or slow-cooked. Serve with naan (microwaved) or rice. A rice cooker might help.
Instant noodles plus ham, sweetcorn, sliced boiled eggs etc. ("Cheat's Ramen")
Home made pizza is healthy and can be eaten cold or reheated. If you get a kit it's not too difficult to make.
If he likes eggs and ham, try German schinkennudeln - that's a popular classic in our house. wholefoodrepublic.com/german-ham-and-egg-noodles-schinkennudeln/
When DS was this age he also had lots of picky dinners, cold salad type stuff with crackers, cheese, ham, etc. You could chop everything in advance and put into little tubs to portion out.