' He will not go out anywhere with his father, if his mother doesn't go too so I do not think it is anything to do with his parents living apart (which they have done since he was a baby).'
I find that sentence confusing, if he won't go out with his father without his mother present too then EITHER there is a problem with his dad OR his mother is overprotective AND/OR this little boy is very good at manipulation and you do say he's very clever.
If I were his mum I'd start sending him to his dads on his own. I think if mum and dad take back control and stop giving in to his demands it will help the current behaviour, it sounds like he's got some ultimate plan (perhaps of mum and dad playing happy families on a more permanant basis?) and has upped his game because his current tactics aren't working.
Do you think maybe his mum and dad are hanging onto hopes of the same? Because I'm suprised his father goes along with it.
I don't think thats really what you were expecting but I hope it helps.
It seems that my grandson is/was being bullied at school (quite badly). He has now opened up to us about what has been going on, so his Mom is trying to deal with it along with the staff at the school.
His behaviour has improved for now, and if he starts to 'act up' again I think it will be an indication that things are happening again.