Ugh, I feel your pain!
My daughter was able to cook some foods for herself at age 9 so your son should be able to as well. My daughter could make a sandwich, mac and cheese, toasted cheese and some other foods.
The rule in our house is, you have to try what's been cooked. If you don't like it, then you don't have to eat it, but no one is making anything else for you--you'll have to do it yourself. AND you have to clean up after yourself.
I stopped buying junk food so if she has to make something for herself, there aren't many bad choices she can make. My daughter likes to make mini pizzas. I buy bagels and pizza sauce and she spreads on the sauce, sprinkles on cheese and bakes in the toaster oven.
My daughter took advantage of the "make what you want" rule when we first made it. She thought it was so neat that she could choose her dinner (after sampling what was on the table of course) so she made herself pizzas and grilled cheese every night for a week.
But then the novelty wore off. She didn't like cooking in the kitchen after we were sitting down eating. She didn't like having to cook and clean up after herself. Suddenly the food we were eating looked a lot more delicious!
I think you can allow him to have pizza every night if that's what you need to do. I guarantee, the novelty will wear off and he'll be eating what you eat. Right now it's just a battle for him.
I would almost suggest giving him pizza for every meal! My daughter used to LOVE eating Ramen noodles and would beg for them. They are so bad for you so I refused and had to listen to begging. I let her have them once in a while. Finally I got sick of it and let her have them whenver she wanted. She ate them for 2 days and hasn't eaten any since! So there you go :)