My gut reaction is to not tell him right away--I think it's a real blow to most kids if they learn that one parent has betrayed the other in that way.
My son's best friend's parents divorced when he was just out of high school. His dad pretty much packed up and left--it was all really quick. The son really went through some rough stuff. He cut his dad off emotionally for 10 years, blaming him for the breakup. The son had started college but dropped out and became a ski bum for a while (he eventually went back to school, but it was years later).
Well, just a couple of months ago, my son told me that this friend was up golfing with his dad--I was surprised. My son explained that one night the son and dad had a real heart-to-heart, and the son learned that the dad left because he caught the mom having a lesbian affair. For years the dad suffered the rejection of his son because he didn't want to divulge the reason for the divorce. On one hand, I thought that was incredibly admirable, but on the other hand, I wonder if the son would have been better off knowing the truth? Maybe he would have benefited from a good relationship with his dad; maybe he never would have dropped out of school. Who knows.. it's all very complex.
So, I think there has to be a way to be prudent with the truth, and causing as little trauma as possible.