essbee, hugs. You obviously care so much. Don't feel guilty. Whatever ups and downs your ds has had, by leaving your dh you are building a stable, peaceful future for him. You are the one who is making that stability a reality for him.
I guess you've already talked to the SENCO at school? I think you should fix an appointment to see them asap once term begins. I think you need more support. The support from our school has been a lifesaver for me.
I've read through janh and soupdragon's messages. I get some of that type of behaviour from my sons at certain times. They are definitely more naughty when they sense my attention is not on them - and remembet my oldest is a mighty 10 years now.. If they had been at the summer camp, general over excitiment, plus lack of sleep (I think they would have found sleeping in a tent so fun they would have slept very badly) plus me chatting to lots of mumsnetters (as I naturally would have done) and not concentrating on them, could easily have tipped them over the edge. TBH this was one reason I didn't go.
They also are more naughty with me than with anyone else..... To give an example. Two weeks ago I took both sons to church. There was no sunday school so they were with me. They were not happy to be in church. The congregation contains many teachers and parents they know. At school, when they have a church assembly they line up, they don't fidgit, they are quiet. I have seen them behave impeccably time and time again. But on this sunday, with me, my 10 year old lay down horizontrally on the church pew, wouldn't join in at all. Kept saying he was bored and kept talk. His brother was intent on climbing under the pew, climbing over me, talking to the people nearby, banging toys around and both boys kept up a playfigthing, teasing game - lots of prods, pokes, pinches and kicks - not vicious but really, really distracting. I couldn't stop them.
This sunday, we went again. DS1 was in the choir (his first time), respendant in cassock, at the front of the church, sitting next to adults. He was a totally different boy. Joined in, didn't put a foot out of place. He really enjoyed it too, he said afterwards. Ds2 was with me. Lots of noise and fidgiting. Grabbed all the nearby cushions and tried to make a tower he could sit on, ran down the length of the aisle during the sermon. I took him out of the church, we went back in, he did the same - totally defiant.
I don't know where this is leading - I'm rambling a bit, but IME, as a mother I get it hard. I get the very worst of their behaviour. But it's something they can switch on and off.
However I get the impression that your son is also feelng very unhappy at times. I can't say that unhappiness is at the root of my sons' behaviour. It doesn't appear so anyway. They are for the most part, cheerfully, smilingly naughty and often quite distractable. So I do think my experience is different to yours. For what it's worth, I really think you should emphasise this aspect to the SENCO or Ed psych when you talk to them.