I would say there's no harm in letting him grow his hair long.
Preventing him from doing what he wants with his own hair is only going to make him want to do it more. Eventually, he'll be able to make his own choices anyway, and the more he's been held back from expressing himself as a teenager, the more he'll rebel when he gets a bit of independence.
Trust me, I know.
I had a mohawk for a short while when I was around 15, and my mom also didn't approve.
In the end, she pressured me to shave it off, so I looked like Billy Corgan till my hair grew back.
Anyway, as soon as I left home I experimented with some really "out there" hairstyles...
I ended up getting my hair 'styled' by the two individuals in the picture below.
Is that what you want for your son?
Now I've kinda grown out of the whole thing, but it was a phase I felt I had to go through one way or another, in part because of my upbringing.If you get too protective over your son's hair, the same will happen to him... Or worse!
Just keep it in perspective. It's not a face tattoo or something permanent, and long hair on guys isn't as uncommon as it might have been before...
Just look at Jared Leto for example!
