mari, I can understand how difficult it must be to see your son in pain and wanting his family back. Many of us would walk through fire for our children, so how is this any different? But it is - because imagine what will happen to your son if you let your WH back into your lives and then he walks out on both of you again. Not only will you be devastated, your son (who might be a teenager by then) will also be incredibly hurt and angry.
If I were in your shoes, I would proceed with the divorce and tell your son that if dad makes things right, you can always get remarried to him (which is true). How your WH behaves during and after the divorce will tell you if he really cares about his family or as Muggle says, if he's just trying to ease his pain.
I worry, though, about your statement that he didn't end things with AP until after you told him that was a boundary. I think that should've been obvious to him, and it's the kind of thing he would've done on his own if he had truly changed. I'll also echo Bigger a bit - your own feelings matter, and they can change a lot in the years to come, even if you try to reconcile, even if your WH does everything perfectly from here on out. I can say from experience that loving someone who was in love with someone else and acted on it for a long time is not easy to sustain. My feelings toward stbx degraded as the months went on, even as his feelings for me started to return. Just knowing how completely he had stopped loving me eventually destroyed my love for him (and this was 2 years after dday). Don't discount that in your own situation, too.
R is a two-way street, and if you don't want it, you don't have to sacrifice yourself for your son. Get him a decent therapist, continue to support him, and in a year or two, he will adjust, and one day when he's much older, you can explain why you had to leave his dad and he'll understand.