Actually, you might be surprised - my experience since diagnosis is only as an adult, but it's quite surprising how much is actually out there. There are the charity groups, but also things like guided walkthroughs at airports, call centres being more patient etc. Hell, even restaurants are willing to bend a bit and find quiet tables for us, and all of these are happy to be discreet about it.
@Whenyourefifteen - it could be worth pointing these things out to him, suggesting that the benefits of a formal diagnosis extend past the school environment...not least, because it'll allow him to forgive himself for the ways in which he feels like some sort of weird alien (that's my terminology for myself as a child...I wouldn't suggest using that with him unless he's described himself as such, obviously).
There's also the fact that, given a formal diagnosis, he doesn't actually need to act on it with the school - he can just keep it in his back pocket as an emergency "Hey, teacher, I'm really struggling with sensory issues today, could I be excused to the quiet room for a bit?" option. The school will almost certainly be glad that he doesn't require constant accommodations if that was the case, and as such I'm sure they'd be happy to work it in.
Personally, if he's thinking of going to university, I'd absolutely recommend it - when I lost my familiar environment and friends group in the transition to university, I completely fell apart both academically and as a human being. Even if he doesn't want anything from it now, the time may come when he really needs it...and the waiting lists being what they are, by the time he does need it, it'll be too late. So maybe you could sell it to him as future insurance, just in case?
The fact is, though, it's extremely difficult to formally diagnose an uncooperative patient - so your only real options are to sell him on the benefits, wait until he comes to the conclusion he wants to do it, or not do it at all.