I was a nursery teacher for many years and I would have flagged this up to you too.
It’s not just that he’s not playing with others, you say he isn’t playing much at all - so no parallel or independent play. I wouldn’t be surprised by a child not playing WITH others but one who didn’t play much at all would stand out.
You also say ‘doesn’t want to join in with the activities’ - does that include the adult led activities eg carpet time, story time, adult led games etc?
I absolutely wouldn’t be leaping to any diagnosis or assumptions about neuro diversity, but depending on what else they are seeing I wouldn’t be ruling it out either. It could just be that he’s not settling, feeling insecure, wants the reassurance of being near and adult, isn’t too sure what to do with himself there yet.
I’d try and have a call or a meeting and ask them what their suggestions are other than extending his hours, as that’s not going to work for you just now. Ask them what specifically they are doing to help him settle and integrate more, and how you can support this at home.
I’d definitely look at some other opportunities for socialising with other children outside of school, but also building up his independent play too. My daughter is, and always has been, very sociable but she’s also an only child and we’ve really had to work on encouraging her independent play recently. She’s 5.5 now and I realise we didn’t do enough of it earlier.