I didn't start putting a topknot on Princess regularly until she was about a year. That was the first age she'd allow it.
I think they look super cute with or without. And I think boys look just as cute as girls with one.
You could start to grow it out & try it to see how he does with just a band for a while. But you might have to be persistent to see if he'll accept it and if not you can clip it off.