Hey monk. Here are my thoughts: 1. Affairs are a bad idea. You are disrespecting yourself, your wife, the other woman, the other woman's spouse and family, and your kid. If a f%ck worth all of that?
2. Get the SSM book, get Passionate Marriage by David Schnarch, and go to counseling by yourself.
3. An other book that might help is Boundaries in Marriage. I just finished it and it does a good job of explaining a course of action you might want to take to get things back on track.
4. Do all you possibly can to save the marriage and make it into something with which you can live. But if, after all that, the only reason you're sticking around is because of the kid, think about how he's going to feel when he's 20, you get divorced and tell him that he was the only reason you made your life miserable for the past 20 years. Remember that kids learn by example. If you're not setting the stage for a loving, affectionate, involved, healthy marriage when you are in his presence, you might be doing him more harm than good by sticking around. But remember: you must do all you can to get to that healthy stage with your current wife.
