I agree Saintly... yes, set those boundaries but maintaining the kind of language you would use with any other adult e.g. we're eating here now but if you don't want to join us and choose to do your own thing, that's fine with us.
I just saw your post about compulsions there, and I think you are right, they are misunderstood as being what the person wants/is choosing to do when actually it is what it says on the tin: a compulsion, and often terribly unwelcome.
Off topic here but it slightly annoys me when I have seen this categorised as "self-stimulatory" behaviourin autism lightly, where it is suggested that watching the spin cycle gives a sort of "high" that the rest of the environment might match, because this isn't true universally and actually different behaviours have different functions for different people. In your son's case, it was less of a "drug" giving a high than something he just couldn't tear himself away from and needed support to overcome.
In the same way anxiety is a term that is used to describe a lot of different things... I do think anxiety in autism is very often sensory, but not in the sniffing/scratching way more like, anxiety because the brain doesn't react the same way to social stimuli so what seems predictable to the rest of us is more anxiety-provoking because it isn't as easily understood/categorised/generalised. Perhaps neurological is a better word for what I'm talking about. Whereas my anxiety was very much learned more from the environment e.g. life was unpredictable growing up. Although with theories of brain development now, there is an argument that having an unpredictable and chaotic childhood can really affect how your brain functions so I am more likely to feel anxious around unpredictability as an adult even though things have changed in my life just because my brain was wired to treat it this way when I was a child. So different experiences and neurological tendencies can have broadly similar results, meaning an OCD adult can seem very much spectrummy even though there are different things at work, as there is also much overlap.
When I had compulsions to clean etc I often rationally knew, as I got better, that it was a useless, pointless behaviour that wasn't fulfilling its function (e.g. it wasn't making me feel less anxious and/or reducing the risk of something terrible happening) and yet, despite being a very intelligent, logical scientific sort I actually couldn't stop myself from doing it at a behavioural level. I had to talk myself out of it like I was talking to a child, it was absolutely WEIRD. And as you say, that isn't there for a child who is preverbal (and it's hard to know how it could be made to be) so it does have to be behaviourally enforced by someone who, well, can do that. And perhaps with an adult, we need to challenge it by pointing out, gently of course, that it is irrational behaviour (while giving a choice to opt out too, because as you say direction would be inappropriate).
I am just thankful that my experiences were a short-term thing, triggered by hormones and something quite treatable (functioning mainly as avoidance, so once I stopped avoiding they just disappeared). I really do feel for people who live with this long term, for whatever reason they have this. It's really no fun and quite frightening to think your brain can be so out of control.