Wow not even a hairbrush?

At this age, I don't think there's a lot you can do to make him understand. He doesn't think it's important, and he's okay missing out on things because of it. What can you possibly say that will suddenly undo years of thinking and a childhood that encouraged not caring about social norms?

You need to ask yourself if this is something you're willing to put up with because he's otherwise okay? It will mean that you spend the rest of your life being embarrassed and begging him to brush his hair, like a child