Thank you all so much. Deaky and Living, I'm so sorry you have had this sort of crap to put up with. It is hard to know for me how it affects a child in ds's position. My parents were both there for me...ex's dad wasn't around much so maybe that has a bearing.
Have thought an awful lot and come to some conclusions - some are the same as what you have posted. You got there before I did, these things take me a while to process (have known this man over 13 years)
Basically when it was just me and him, he treated me this way - turned up on the wrong day, didn't turn up when he had said he would. He changed the plans constantly, because he felt he had to I think, total commitment phobia. I don't think he even knew he was doing it really.
One time I pre-empted him, knowing he was likely to dump me (he did that a lot) I turned up and said, 'You're going to dump me aren't you'. To which he looked absolutely shocked and blustered 'No of course not!' ahem.
So if I thought one thing, he had to make it another.
He dumped me the day after instead 
If it were just me, I would go no contact, because otherwise you are totally stuck in the trap of 'I love him, I want to see him whenever I can' while he is merrily playing hell with your feelings and arrangements. I did that for years 
I think when someone loves you, they are there for you. He clearly didn't love me (though he said he did) it was all just part of a big drama in his mind - I served a purpose but I still don't know what it was.
Ds is the same - he serves a purpose to his dad but no one, including his dad, knows what that is. I don't think he does love ds.
Not really.
He has done this before to other people, other children.
He is always there for his step children and his new wife.
That's gotta tell you something really - having offered to bring ds to him last week, having been open to changing the day, the week, whatever - he still finds reasons not to see him. It's like, ex is torturing himself (and ds) on purpose as that's what 'love' is, to him. Not being able to see someone.
I think boarding school may have had something to do with it 
Anyway as it isn't my relationship, but ds's, I've explained to ds that what his dad is doing is wrong, and unfair, and hurtful, and that ds is worth more than that, but I don't want to make the decision for him as that wouldn't be right? I don't honestly know what's right.
Upshot is ds doesn't want to see his dad 'for a while'.
How can a 10yo make this sort of decision - I don't know. If his dad does contact me today he is in for a hell of a fight.
I intend to start with 'What the hell do you think you're playing at' and go on from there...he will apologise and expect to be invited to see ds after school. Not happening.
Regarding the Granny, I have no idea what she thinks at all, as I've never been allowed to meet her 
It's all bloody mad.