Write, luv,
your ex's values may appear to shift constantly if he invests a lot of time and energy deliberately doing / thinking / telling himself he wants exactly the opposite of whatever you [or other targets, most likely including your son] happen to want / believe / enjoy / value.
This is called 'countering'. Patricia Evans describes it as one form of abuse... I've known people who do it, though, and at least in my own experience it's not merely a behavior; it's a lifestyle, a calling, a vocation, practically a religion, for the people I've seen who are really into it.
I haven't met your ex, so I could be waaaaay off base, and I know that. But if he does a sort of amazing weathervane thing, and the only consistent thing about him is that he can be counted on to be in disagreement with you - or your son - no matter what,
then your son could definitely benefit from learning about countering, double-binds, and crazymaking in general - because it will help him see that there really is a core, stable value there. And that value is: disagreeing with whatever you or your son say, do, or think. Period. No mystery, no deep insoluble problem. Except, of course, the eternal mystery of why people who do such things consider such behavior to be loving.
That crack about 'nascent personality disorders' - you mentioned it earlier elsewhere - suggests to me that your son is becoming the new target, in your place; he's vulnerable and easy to get at, and forced to be in proximity. It also suggests to me that your ex is at least partly aware of what he's doing.
It's hard to believe a person can virtually center themselves so much in 'the act of opposing' that this becomes almost all there is to them, but I've seen it. Way more often than I wish I had.
Edit in: logged out, shut down, made coffee [I'm taking a day off work, hooyah] and realized this sounds a lot more hardline than I intended.
I don't mean to imply that your ex, if he's like this now, has always been like this, to the same degree.
I think this stuff is progressive, gets worse over time.
Someone who gets their jollies out of 'winning' - as though every conversation was a contest - if that's what's underneath this - usually does get worse over time.
They get more and more competitive about less and less and less, until eventually all you have to do is say 'good morning' to them, and you'll get slammed with a two-hour, 120-decibel diatribe about how it isn't morning in all time zones and it certainly isn't universally good.
I've seen a few folks who were very far along this path, some of them sadly not even out of their 30s yet. Mostly male, but I've seen it in women coworkers too.