My daughter struggled with this a lot in Reception (pre-Covid - she's now in Y2). She is now OK with familiar people if she has seen them recently. Lockdown has definitely made things harder (e.g. with grandparents who she hasn't been able to see in person in ages, she's now unwilling to participate in Zooms, which she would have done earlier on - but I think sadly they're just too unfamiliar now ... hopefully we can fix that soon!)
I didn't (and don't) put any pressure on her to say hello to adults etc. Just modelled it for her. I do now give her praise if she manages to talk to an unfamiliar adult or child - but I keep it very gentle/limited because praise can feel like pressure too. I might just say something like "that was a nice conversation". For DD any pressure is counterproductive: it just makes things more anxious and makes her less able to do anything due to overwhelm of the feelings of pressure/anxiety.
Re: other children, one thing that DD's school did in Reception that was really helpful was that they had a lady come into school once a week with DD (I think she was in fairly regularly with other children too for different issues) and just encouraged her to interact more with the other children at lunch or break. They called it "scaffolding". So e.g. if DD was playing next to (but not with) other children she'd just say "oh look, Jonny is building a tower too" or "I wonder if that piece next to Amy would be good for your tower" or "oh Sam is pouring, I wonder what it would be like if you both poured water at the same time?" or whatever. Very gentle, no pressure. It did seem to help DD to just notice what other children were doing and have the confidence to get involved more.
The lady didn't have any special SN training but she was a nanny who had worked with SN children before, and I think she just "got" how to be gentle but also give a little encouraging push.
I think also just doing more interactive play with you is always good. I remember at about that age DD and I would play being Winnie the Witch and pretending to turn things into other things. She loved Winnie the Witch at that age, and was slightly obsessed with sticks (which we'd use as wands among other things) so it was engaging for her, and helped practice turn taking. We always found with everything we could never leap straight to doing it with other children. Everything went that DD got comfortable doing something with us first, then with other trusted adults (like her TA or this lady who came in to school), and only some time after that with other children (and even then generally in small groups where she'd had plenty of time to settle in).
It sounds like your DS was doing these things before to some extent so I'm sure he hasn't lost the ability in general. There's probably just too much else going on for him to be able to manage it at the moment. If he's only recently back to school he probably just needs more time to get comfortable by himself and with other adults first, before he'll feel "at home" enough to risk interacting with the other children - so hopefully he'll get back to doing them quite naturally as he settles in and his anxiety subsides a bit. But maybe a bit of extra TA time or supported play could help along the way. Or given it's nearly the holidays, maybe some play dates over the Easter holidays so he can practice a bit in smaller groups, and not be starting from square 1 again after the holidays - or even just supporting him to engage more with other children when you visit the playground? And then school might be able to do a little bit of scaffolding with him during play when he gets back?
Is it even possible that he is still playing with them at break/lunch, but just can't manage it in the morning when everything is so busy and he's got the transition from you to school to deal with too?
The good thing is children at that age are very flexible and don't hold a grudge - so even if they think he's not interested in them now, if he starts to be interested and play with them again in a couple of months I'm sure they'll let him in and he'll be able to get back to where he was.