Have any of his teeth ever hurt, or does the dentist catch them before they cause pain? Pain CAN be a teacher, but it isn't always strong enough to have them remember the lesson when the tooth does not hurt.
Can you keep sugary foods out of the house or in a lock box in your room to be doled out sparingly? Will it cause him to hoard food and/or eat only junk food when he eats out of the home? If you can move toward having much less sugary stuff at home that might help. Of course, since the lack of brushing/flossing is the main reason, it may be that any food will cause the cavities. In that case removing the sugar and junk won't make a big difference.
One of the things that makes a difference in boys is girls. That was the ONLY thing that got Wiz to brush. Not even my mom's demands and refusals and over-policing made a difference.
I refused to get braces for him. They were largely needed for cosmetic reasons, not for health reasons. I saw NO need to pay for them if he didn't care about his teeth. Not even the threat of being reported for child abuse made a difference. (I actually called and asked if not getting a child braces was endangerment. They laughed and said that if a medicaid dentist did not require braces they were NOT required.)
Sadly, other than daily reminders/checking up on him, there isn't anything YOU can do. Just like getting our kids off drugs, we simply cannot force them to do certain things. You could address any sensory problems (I am sure you have already), add incentives, etc.....
Probably the ONLY thing likely to make much difference is to force him to pay the bill. If he gets allowance, dock it. If he gets birthday/xmas money, dock it. Keep the tab running for however long it takes to pay it off. If he wants a new video game/book not required for school/sports team fees, make him pay for the teeth out of part of that money. Not all, or he will lose incentive.
But if he wants a new gameamabob and it will cost $100, plus $30 for a game, he can only buy it when he has $260. He has to put HALF of whatever $$ to the teeth. I would do a running deduction, not have him save $$ and give you half before he spends it. Make him give you half as soon as he gets it.
If he has lunch money and is skipping lunch, esp if it is to buy something, make him give you the money back. This may encourage him to eat during the day, even if not hungry. It will also keep him from using his lunch money as a source of cavity-tax funds. This is partly to help him see how important it is to eat at lunch and partly because if he eats at lunch he may snack less in the night.
Does he ever use his spending $ to buy candy? I would put a higher tax on that money. If he has money for a 50 cent candy bar, then he must pay you a dollar before he can eat it. Also to help make the link between the cause of the cavity and the consequence of paying the bill for the dentist.
Half is just a number I created for easy math. ANY number you want to work with will do.
I am sorry there are so many issues surrounding his eating, esp as it impacts his growth.
And esp because it is so hard to get them to change. I suspect that when he is older he will really regret the damage he is doing to his teeth. (He will also regret helping the dentist buy that new boat, LOL! - Our running joke with my dad's dentist is that it is time for dad's checkup because W wants a new boat. My dad has a filling, crown, or other major problem with every tooth in his head.)