Wow - it's no wonder you're looking for diversion. But the affair is not what's really going on here, despite its immediacy -
The real issue is between you and your husband, and it's obviously the issue of how and why you two have allowed his son to disrupt your marriage. I say you two, because you have, despite your protests, allowed this to happen as well.
One thing that struck me is how passive you have been in all of this. You say HE decided and HE sprung the news on me and WE bought him a new truck, and WE sold my car -- didn't you have a voice in these decisions at all? This kid is 18, capable of college or earning a living, but you two are supporting him. If you protested, what did your husband say? How and why did you contribute all of your savings and retirement to your husband's legal bills concerning his son? Did you do this as an act of love, or because you fear standing up to him, or some other reason?
I think marriage counseling for you and your husband would be the best thing you could ever do. Your marriage has never come first since day one, and if you want it to survive, now it must.
Good luck -