Nthing that you should let him get what he wants... it's been a long time since I was a teenager, but that was when I first decided I'd kind of like to have a kilt (at the time I don't think Utilikilt and other non-trad kilts were even around) and I would have been super disappointed to get a solid black garment instead. I'd look at a USA Kilts casual; low-maintenance (fine for just knocking around in; teflon-coated and machine washable) but it also dresses up nicely, to a point. And it's made to measure.

As for acceptability at school, I think that a more traditional tartan kilt would actually raise fewer eyebrows than a solid-color modern kilt, both from an administrative and peer standpoint... but regardless, I'm sure your son knows he's going to stand out in any kind of kilt. Not necessarily a bad thing if he's prepared for it!