I think he sounds like he is trapped in an abusive relationship but has no understanding of the dynamics of it, or why he puts up with it, and he might not even see it as abusive.
He might be interpreting emotional turmoil as evidence of "love".
For some reason, on some level, he might unconsciously be inclined to seek out an abusive relationship (e.g. he might think he can save her, or if only he had tried harder she would be nicer and so on). Do you know anything about his background? If he grew up in an abusive household he might think it is normal and not really see how bad it is.
A lot of abusive people try to tie the knot quickly (so they can have even more control) so that is no surprise, I wouldn't really take it as evidence that he didn't care about you.
He might also be suffering from a degree of Stoklholm Syndrome, where the person who is entrapped and abused by someone ends up taking the side of the abuser, even against themselves (for example they might have thoughts like "they are angry, I must have done something wrong")
But for your own health's sake trying to get some emotional distance would be good as this rubbish could go on for years. He is likely to let you down over and over again so long as he is with her, as he will put her demands above everything else. He has to really want to get out of the situation, and go for some serious counselling or something similar in order to sort himself out emotionally before he would be ready for a relationship.
But since you have a son it is not as easy as cutting contact. Can you contact him just by text or e-mail or something for a while, if you find that less stressful? I agree with you about not wanting her around your son as she sounds off her head, and I would be worried about her being mean to your son. Unfortunately it will be difficult to keep her away from your son in the long run. I will look up some links for you about men in abusive reationships in case they help.
I was in a relationship with someone who had left an abusive marriage, who went ballistic when he told her about me. He stood his ground initially but she got nastier and nastier (they had children together and she stopped him seeing them and did some other nasty things) and eventually he broke up with me, mostly from the stress of it all as far as I could see.
Like you we had an easy-going relationship and we had not even had an argument, ever! He asked to remain friends (which was a bad idea for me as it happens as it was too confusing) but then out of the blue he cut me off after a few weeks without explanation, and stopped answering texts. I was hurt initially but I now realise it was really his issue and not mine.
She may have made it a condition, or he may have gone back to her, I have no idea. The last time I saw him I could see very clearly that he was being played, but I don't think he could see this. With a cycle of abuse there can be such relief when the abusive person seems to be behaving ok and "nice" that the abused person can almost forget the bad times, or think this time if they do everything "right" it will work out ok.
In reality the abusive person will always find something to complain about, some new thing to find fault with, or new way to be manipulative so it is never ending. Personally I always find myself more nervous when I am around a volatile person when they are being "nice" as I am waiting for the axe to fall, and sometimes they up-the-nice in order to increase their powers of manipulation.
Anyway not sure if anything I have said is helpful. At least for me I did not have a child with the person, and it wasn't as long a relationship as yours.