Oh this must be horrible for you. So from what you're saying, he was having an affair with an old flame while you were pregnant with your second child and left just 4 months ago?
You've probably read on these threads about when some people have affairs, especially the ones that leave home, they simply have to re-write history and pretend that if it hadn't been for their affair, they would have left anyway. Hence you are hearing absolute bollocks about you "pushing him away".
He's going to be in even further in denial now he's decided to get married to his old flame, so I honestly don't think you'll get any truthful answers from him just yet.
In these situations, what I recommend you do, is to work out when contact was resumed with this woman on FB and recall precisely, using key events as milestones, how your relationship really was just before he made contact with her.
IME, betrayed partners who do this timeline, discover that it was the affair that caused any distancing and unhappiness and that this was created entirely by the person having the affair.
Don't let anyone take away your truth or your more accurate memories. But remember that whenever human beings do something that they believe to be fundamentally wrong, they have to find an excuse for it. The weakest characters tend to believe their own lies and so what you are seeing is simply a huge dollop of history re-writing. How else can he defend having an affair and leaving you, his 5 year old and a 5 week old baby, for an OW?
Once you work out that there was nothing about you or your relationship (this was all about him) AND you reclaim your more truthful memories, you might feel a little better.
However, you are only 4 months in from a massive bereavement, so do consider getting some counselling to help you process your feelings.
WRT his intended marriage, unfortunately affairs with old flames tend to be the most intoxicating of all and of all affairs, tend to become the longest-lasting relationships. However, given that he has already walked away from one family and exited via the infidelity route, his OW will always wonder when it will be her next, especially if they have children and she becomes vulnerable like you were, when you were pregnant. Likewise, if she left her own relationship to be with him, he might be forever looking over his shoulder too.
Concentrate on you and your lovely family, but don't let him rob from you, more than he has already. Don't let him take away your truth too.