Hair grows back. At least it's not a tattoo or a piercing.
That said, he seems a little young to be pushing the limits like that. The parent is the paying customer. It's really their call whether to allow it or not. I'd try to find out what the motivation is for wanting the mohawk look. It's not like he can change his mind and fix it easily if he doesn't like it. Since Halloween is coming, maybe you can find an inexpensive Mohawk "wig" that he can try to see what kind of reaction he gets (assuming it's a reaction that he's looking for).
Maybe you can come up with a kind of compromise in the meantime. I've got a whole box full of different colours of wash-out hair colour. It allows the kids a measure of self-expression, without creating a huge distraction for the classmates/teachers.
Doing stuff with your hair can be kind of fun at that age, provided that it's not too extreme, and can easily be reversed. Kids that age are notorious for changing their minds more often than they change their underwear. Once he's more inclined to live with a decision like that (3 months is an eternity for a teenager) then you could try things like changing the hair colour and using cool products to make it spikey and stuff like that.
They do actually grow out of this. Our house rule is that we won't allow tattoos or piercings (except for the girls ear piercings) before 18. When they are 18, and can finance it themselves, they can do it if they still want it.
Edited to add: I've just read the posts about the boys not wanting a Mohawk, but the mother cutting it that way anyway. Sorry, but that's just wrong. Why would she want to risk her kids being stigmatized, especially if it was not their choice? If the school was aware of this, they may even be inclined to inform CPS because it's borderline abusive.