Cherryface.....like some others have posted, it really is a vast spectrum. Autism is diagnosed using the triad of impairments and my son fitted all of that back then, aged four. He is definitely more Aspie (high functioning) but all the same, it is all ON THE AUTISTIC SPECTRUM.
I recently attended an NAS "Help!" course and was so sad to sit with parents of teenage kids who'd gone through such difficulties without a diagnosis until age 13, 14 even . They had spent a huge part of their school life just trying to survive and "get through it". A diagnosis, as you say, is key....for him, for you and for what's left of his schooling. Can you ask to be referred to a paediatrician who can make a diagnosis?
As far as school goes has he been seen by the Educational Psychologist? CAMHS? CASADT? He "isolates himself"?? Don't school want to investigate why? They should. You may need to toughen up there, Cherryface!!
My son would often isolate himself because.....it was just SO HARD trying to do the little things that come naturally to neuro-typical kids. He knew he was different and often took himself off because it was just easier. He always looked excrutiatingly uncomfortable when anyone was trying to engage him in a conversation or play. He'd often "blank" the situation (just glance away - like he could shut it out). He manages to "engage" better now, but then he's ten and bright. As for memory...as part of his Statement assessment, the Educational Psychologist said that he had an exceptionally poor working memory. So now we do one instruction at a time..... He can't do 3 or 4 requests one after another without prompting but.....he has an eye for detail that astonishes me and recalls the smallest piece of data on his "chosen subject". It's a funny old spectrum!!
I think your son needs assessment specifically for an autistic spectrum disorder and you may find that you need to bang on a few desks to get it.
Two good books I've got....
Tony Atwood "Asperger's Symdrome"
Richard Lavoie "It's So Much Work to Be Your Friend"
The latter is excellent for unravelling the social thing.
Empathy? When my husband left a month after my son's diagnosis needless to say, it was a hard time for me. At age 4, he would come and sit on my knee and stroke my hair and give me a tissue because "you look sad, mummy".........enough said.