I know this thread has been rocky at times, and I'm sorry for all the OP's distress, but can I just say it's been one of the most powerful, passionate, experience-packed threads I've seen on here? (And I'm a veteran of these sorts of threads.)
I wanted to add my own experience, maybe not so much for the OP now, but for all those other people who'll read this in future. It will be long sorry ....
My experience (ds, now 11) is of perhaps a rather classic 'borderline' child - maybe closer to rose's son's presentation than some others? Ds is lovely at home, and indeed anywhere that life can be 'on his own terms'. He's a dream kind big brother - because dd (10) happens to like and joins in with his very extreme precise imaginative games. He's no trouble to us - because he'd rather entertain himself than demand attention/interaction. But school is - and has been since concerns were raised at nursery - a different matter.
When he was little, I had no idea that what I thought of as ds's happy "self-containedness" as a toddler (no tantrums, passive go-along-with-everything, no stranger anxiety, no endless whywhywhy, no mummy mummy mummy) should have been a concern. We all convinced ourselves that he didn't follow the group at nursery because 
he was "too bright" to be in the herd. We just didn't see that, to put it very simply, ds was missing some fundamental instinctive behaviours - and as has been said, that is what matters.
Here is the crucial bit for rose, though: we did take the hint, and did get him assessed - and the outcome was no diagnosis, no 'label', no conclusive explanation. BUT that did not mean ds does not have problems, and real ones - I've just done the Aussie Asperger's link below and he ticks just about every box, plus some - and that school is not very difficult sometimes. The fact is that the more borderline presentation becomes a problem LATER, let's say after about age 7, and if you haven't got strategies in place by then the issues/traits in the child are even more fixed and you have got a bigger problem to deal with.
So I would say (a) do please get him assessed, and as soon as possible (I realise rose is now planning to do this, so this is just encouragement) but (b) IF the answer is inconclusive, please don't assume everything is thereby and therefore fine - keep an eye; take every bit of advice and input you can.
My experience might suggest, I think, that the borderline children are almost in a way more difficult to deal with than the clear cases - because every borderline child is borderline in their own unique way, and the variety of strategies and input you need has no clear pattern or route to follow. So you need everything you can get; and you need an attitude that says you will make the most of everything that is offered as help.
I'm sorry to have gone on so. What I also thought I might do is add a list of what has turned out to be ds's actual day-to-day problems now he is older, so stuff that rose might perhaps want to look out for. Would that be useful?