Glad you had a good afternoon with him, i feel so much better and calmer when i get a temporary relief from the behaviour.
Do you have a child health GP at your surgery? If so, i would raise your concerns with them (if nursery have raised any issues i would tell the GP that also). The GP would be able to refer you to a community paediatrician for assessment and/or CAMHS (who also consider whether there may be a condition responsible for his behaviour.)
My DS can also not play with toys and is unable to play by himself but needs either DD or myself to "entertain" him. I think part of this may be that he gets bored easily as he is quite bright. He will, however, spend quite some in the garden making magic potions with mud leaves etc or wanting to produce copy experiments from "Back-yard Science". He also had few friends at pre-school and was very quiet.
However, since starting school he has had no problems making and playing with friends. (I think he was bored beforehand.) He loves school.
Following advice from others on this thread i have looked at the triad, spoken to a close friend and neighbour (who is a paed at GOS) and spoken to someone i know whose child is on the spectrum. They have told me that they don't believe DS has aspergers, although i am still open to CAMHS opinion on this.
My DS, now 5.4, has stated that he wants to see a doctor for a sit down so he doesn't get angry or hurt us. He has also in the last couple of weeks requested a "calm down area" and made it himself last night (with my help) - just a blanket on the floor in the playroom with some books and his pirate ship (chosen a bit randomly as he's never been particularly interested in them before) but i think it makes him feel secure.
Re the time out - do you have a good childminder locally? My DS used to go to a lovely one when i worked - some of the other children would just go for a few hours a couple of times a week so the mums could have some time out.
The other thing that my son likes to do is help me - laying table, dusting, helping me hoover (it's bit slow and only do a couple of paths across the room bit seems to improve his self esteem slightly.)
Just wanted to ask how your DS is when he has time with you when DD asleep. I and other friends have noticed that my DS's behaviour completely different when DD around. Partly why i'm not sure re my son having SN, but more inclined to think attention seeking. For example, all shopping trips used to be a nightmare. Now he's a bit older supermarket with just DS is completely different experience than with both DD and DS.
Like you, my DS does get lots of 1:1 attention but as soon as this stops the behaviour starts.
Hope you get some answers and bit of time out soon.