He doesn't want to.
I would think discretion is the better part of valor. That would mean not posting about the wedding endlessly on Fakebook and other social media. Plan the thing old school, happy couple makes plans themselves, and then eight weeks before the wedding send out invitations. If the ex finds out, he finds out. Common human decency would mean you don't rub his nose in it..
How close is your son to the rest of his father's family? will a lot of them be invited? Are they on speaking terms with the A?
I don't think he should be invited. I do think the less he knows about the wedding, the smaller the chance he will show up and make a scene.