Hmmm, I think it's possible he will be affected by his friends not being around to play with at nursery, but maybe if that was the only case, then wouldn't he be displaying this behaviour actually at nursery too?
i really do feel for you as you are obviously a fantastic mum to be searching so thoroughly for help to sort this.
If it's any help, i do think his behaviour is perfectly normal and i'm sure it will pass. Is the behaviour all day. or just in the morning?
The best way to handle this i feel, is to eliminate choices, continue to stay calm and just ignore the negative responses for now. Try not to get upset in front of him, (i know it's so hard not to), and praise, praise, praise all his good behaviour.
In answer to your question does outside events manifest in such things, i honestly don't know, it's not something i noticed with ds.
Is it possible that you could have one of the bigger boys he used to play with for the occasional play date?
If i think of anything else i will add to the post, and of course i and i'm sure the others will answer any questions as best we can.
Please keep positive, you are clearly a fantastic mummy...