Shield him as much and as often as you can. You'll be so grateful knowing you did. And as difficult as it can be sometimes - don't let your son hear you vent.
I'm saying this from experience. I split from my AH when my youngest was 13, and never a day goes by that I am not grateful for doing so and salvaging some of my son's youth. He's got good memories now to carry with him into adulthood.