I was worried about DS being a bully too, but when I went to playgroup I saw a real bully in action!
I think he had a problem with 'breaking in' to play with a group of kids, or didn't know how to approach them.
He seems to be shy (again, normal at this age), so why don't you take him by the hand at playgroup and say - oh this looks like fun, shall we play in the sandpit with the others? Then I'd sit with him and the other kids passing spades etc, make sure he didn't whack anyone for the first 2 mins, then he'd be fine.

Then I'd blab on about how fun it was to have friends in the way home in the car etc etc etc.