On the road to an ASD diagnosis... but confused about ADD

[CurrerBell]

I haven't posted here for a while... My DS (6 yo) is being assessed for social communication difficulties. After many months of assessments and appointments, where nothing seemed clear-cut, DS has recently been assessed by the speech and language specialist, who has said she thinks he is at the mild end of the spectrum. We have to go back to the paediatrician in a couple of weeks, and it looks now like a diagnosis is more likely.

School have been brilliant, and are already putting things into place to help DS. While I'm so pleased (and relieved) that he is finally being understood more and helped, I'm still a bit confused.

The thing is, although DS does have some mild autistic traits (e.g. taking things literally), he is actually pretty sociable and seems to have no problems making friends at school. He doesn't adhere to strict routines or line up his toys; he doesn't have obsessive interests; and he doesn't have motor problems. Mainly, I'd say he likes to do his own thing, and he struggles with the expectations placed on him by adults. He finds it hard to sit still, and to switch between activities. When he starts to feel uncomfortable, he just loses it and gets very silly. He switches from his normal engaged, happy self to a strange compulsive state where he does things like:

Spins, barges into things, or crawls on the floor
Hits himself on the head
Fidgets, pokes, disrupts others when he is expected to sit on the carpet
Zones out: very difficult to get his attention.

However he can concentrate for hours when he is engaged in something he finds interesting (e.g. computer games, or Lego).

My friend said something about ADD being part of an ASD diagnosis. I've been noticing DS's 'attention deficit' problems more and more as he gets older, and it got me wondering whether this might in fact be his main problem. Can mild autistic symptoms fit in with this? I tried to ask the paediatrician last time but we only had about five minutes with her... she said they'd only look into ADD if ASD was discounted. I'd like to understand more about how to help him... Sorry if this sounds a bit confused, but any help would be much appreciated!