OK...a bit of personal experience here - I'm autistic, and I've been in bands for nearly 40 years.
When I was younger - think primary and secondary age - I was ridiculously good on my chosen instrument for my age/experience (I was as good as most adults by the time I was 8/9), but that didn't generalise to all music. That led to a certain level of "cool" amongst my fellow students and being hailed as a prodigy by adults, but the music teachers not being particularly impressed. As far as I was concerned, that was absolutely fine because it gave me a crucial social entry point which (up to that point) I'd been seriously lacking.
The part which I was still lacking, and it dogs me to this day, is that I love to play in bands, but they have the major downside of involving other musicians. I absolutely can't deal with incompetence, unreliability and lack of focus - and almost all amateur musicians have at least two of those baked in to their core. That inevitably leads to me getting endlessly frustrated with the whole endeavour, and I end up fixating on it (drives my other half nuts). As it is, I've pretty much given up on bands now, even though I love writing and performing live, because my life is generally more relaxed and peaceful without them.
So...in my opinion, the best thing you can possibly do for him with regard to his music is actually nothing to do with the music itself - leave him to it, he'll be able to direct and focus on that perfectly well without needing to be pushed. Teach him how to get on with a close-knit, small group of people - how to tolerate their mistakes, how to compromise, even how to decide what he's willing to compromise on and what he's not, and how to constructively deal with frustration and irritation when it goes wrong (and it always goes wrong). Even if he only associates these approaches with musical activities, they're things that are best learned young.
With regards to music...there is some autism-specific stuff that's pretty important. Playing live is a uniquely weird problem - as you said, noise (even very loud, overwhelming noise) is absolutely fine for most of us as long as we're a part of that noise. However, we're usually not going to be the only ones making the noise - most of the time (unless it's a covers band), there are going to be other bands on the bill, and they're usually terrible. Unless I stay outside before I go on stage, I'm usually ruined from sensory overwhelm by the time we get there - and if I'm not, the stage lighting will finish me off. Shades on stage are, unfortunately, a necessity.
Also, while I get a temporary high after playing a gig, usually for a few hours (which makes it hard to sleep), I'm then exhausted for at least a couple of days afterwards - mainly from the forced social contact that you just can't get away from when you've spent hours waiting to play in a place where there are other people, as well as all of the people who come rushing up to the stage and want to talk/shake your hand/etc afterwards.
Talking of which - my other half will (if present) always get there first, and defend me by (politely) batting people away when I can't do any more and need to escape. That's really important, and I'm hugely grateful for it - that's something you can definitely help him with, because he'll likely not be in a state where he can gracefully deal with them (ie he'll be in music mode, not social mode, and the context switch is very hard to do in that environment).
Sorry for the wall of text...just trying to brain-dump a condensed version of all the things I've figured out are hard for me over the years...