Oh OP,
Of course you are still in shock and disbelief.
How would you not be?.
He went off with a married colleague.
Two family's having bombs dropped in their midst by really selfish people.
I mean this VERY, VERY kindly, but there is NO way he has fallen in love with you again.
That is absolute bullshit.
The sort of bullshit lying toads say because they need to.
He felt so comfortable telling you, his wife that he despised you.
With the greatest of respect, you would be a very silly woman to forget that,..... or to think that someone who says something so completely wounding, is to be believed, that they have suddenly fallen back in love with you.
That he is so obtuse as to suggest he could still be with her, and that that might flatter you, must make you wonder did he ever know you, or you him.
What I think you can rightly assume is that his plans did NOT work out as planned, and he is furious back tracking as his options have narrowed, and presently, you are his best option.
There is absolutely no guarantee that he won't do this again, infact, I would put money down that he will be unfaithful again.
I would consider it a given.
Your choices are, do you want to continue this yoyo back and forth, delaying the inevitable for another couple of years, possibly adding another baby to the mix?.
Or, do you accept this as your new reality, your son is doing well, move forward with a divorce, and give yourself time to heal.
You are young and deserve so much better that this this excuse of a man.