Hi,
My 2.2 yr old son really dislikes other kids his age. If another child is anywhere near him in the playground he will shout at them to 'go away!' or even shout 'No!' at another child who passes by him when we are walking outside.
He has been going to a playgroup once a week for ages and he's ok there unless another child actually wants to play in the same area with him and he will scream at them if they pick up a toy near him (even if he's not playing with it). I'm not expecting him to share or even play 'with' other children, just tollerate their presence.
He actually used to be really friendly and sociable with other kids up to about 21 months old when this kind of thing started. He's okay with babies and older children and he's usually not physically aggressive. I can't think of anything in particular that's happened to bring this on. I guess it's probably obvious he doesn't have siblings and doesn't go to nursery.
I'm quite upset today because I invited a friend round who has a daughter his age (thinking one-to-one home-environment play might help), but he spent the whole hour shouting at her to "go away" and had a breakdown when she tried to play with anything or read any of his books. He even shouted at her, "not girl say it!", every time she spoke (poor thing). I imagine they'll never come round again 
What can I do to help him get over this?