Thanks for your responses, everybody!
Ripeberry, yes, the nursery he will be going to somewhere between age 3-4 feeds into this primary school - it's actually on the school grounds. I will definitely be sending him there, it's just before then that I wondered if it mattered that he wasn't seeing many other children.
Strandedatsea, I meant playgroups where I stay with him. He is much too young to be left by himself at the moment.
Mamsnet, I'm just not into the whole playgroup scene.
Firawla, yes we don't just stay at home all day! We go to a Mums & Tots drama group but there isn't really any interaction with other children there. Although, actually we also go to tumble tots which I suppose counts - you know I honestly hadn't thought of that! He does have to learn to share equipment and so on there. I guess once or twice a fortnight is better than nothing then!
Ladybiscuit, you make a very good point about him learning how to find his way whilst I'm there to guide him. At nursery, I won't be there to help him, will I? Hmmm, I hadn't thought of that. That does make me feel I ought to do more social things with him.
I don't wish to sound like a boastful mummy but one problem we have is that he is very advanced intellectually (and vocally!) and most children he meets of his own age aren't talking clearly or can't repsond to him in the way he wants so he doesn't really understand them and just looks at me and asks "why aren't they answering me, Mummy?" so he tends to give up on them.