Oh! I've spent ages typing a reply, and somehow lost half of it!
I'll carry on.....
Have you tried a warning system, like a 'Traffic Light' system - green for go, and show him an amber colour (eg a card on a keyring or something) say, twice as a warning when his behaviour is getting out of hand, then a red Stop! if he gets inappropriate and this preceeds the telling off or punishment.
If H is involved in this, eg choosing what the 'rules' are, whether he wants red and yellow cards like in football, and he understands when they will be used, it could help him start to identify which behaviour is not appropriate, and learn how to recognise when he needs to change it.
It needs to be and consistent in order to work, and any consequence needs to be fairly quick so he realises what it relates to. (Eg, if I hit Oliver, I go to my room, then he goes straight away. If he waits til you've unravelled who said what to who first, then he wont associate the consequence for the actual hitting)
What about a reward system, to praise all the positives and not focus on the negative stuff. If he responds so well to "Do you want a sweet?" then a simple reward he helps to devise might help.
Just a thought, does H respond better to information/instructions/rules etc if they're writtten down rather than spoken?
If so, who does the writing.. you and the teacher, or Harry himself?
Everyone's different, but it might make a difference!
Also, do you know what way he learns best? Ie, reading, writing, listening, doing etc? The school or psych team might have a simple test which would then give him and the school the best way in which to help him learn
(I'll have a look if I've got it somewhere, its v good)