A different perspective.....
Is it possible that he's just bored and having a bit of an identity crisis? Jumping from $20K to $70K is huge, and can make you re-assess things about your life, asking questions like, "What have I settled for because money was tight, and what do I really want now that I can afford more?" Add lack of direction to that, and it's pretty easy to sort of "lose your footing" in life for a while. You said he was sitting around the house and went to work because he was bored -- is it possible that he just needs a good hobby? (My DH is SO much easier to be around when he's got a good project going, and he's restless and irritable when he doesn't have enough to do.) What does he normally do at home when he's not working? What makes him happy in life? What is he doing when he's having fun? There's definitely something about being home that is not fulfilling him, but I wouldn't necessarily jump to the worst-case scenario about cheating or getting a divorce. Maybe he just needs to buy that old car he's been wanting to fix up, so he can feel important and needed while teaching your eldest about carburetors. Maybe it's time to get that woodworking equipment he's always wanted so he can put his creativity to work. In my experience, men tend to get bored and restless before they cheat, and it sounds to me like he's bored and restless, and perhaps trying to figure out what direction to go now that some things are more financially possible than they were before.
No matter what, you two need to talk! Get to the bottom of his dissatisfaction, and see what you can do to fix it. Running for the door at the first sign of trouble is not always the best or smartest solution, especially when kids are involved.
Best of luck to you.