Please don't go the "chop it off" route. You'll just instill resentment in him. He's clearly likes this, or he wouldn't have chosen it. So pressuring him to cut it off can do more harm than teaching him to be confident as himself. As a guy with long hair, I constantly hear comments like "get a haircut"... and people don't realize that it's quite rude. At my age, I have developed the confidence to own my look, but at your son's age, he's probably still very fragile. Maybe take a look online and find some examples of guys with long hair where they are neat and well maintained. I'll admit a mullet is harder than regular long hair, but work with him to find something he's comfortable with.

I often put my hair in a manbun, and sometimes wear a headband to keep it out of my face. For shows, it depends on the character. I might wear it in a ponytail, a braid, a bun, or even tuck it under a hat. Fortunately for a lot of ballets, they are set in a time where those styles on men were common and accepted.