OP here. I don't plan to file for a divorce. I am trying to understand if there is a way to save our marriage. For many reasons 1. because I still love him and cannot throw out 15 years of life together like that for the goal of splitting the assets.
OP, I'm sorry, this sounds so devastating and complicated. Your thoughts and feelings are understandable. But please know that this man checked out from your romance and emotional part of marriage and chances are that this is forever. Yes, men often come back, after they destroy everything, get old and sick and have zero chances to get a quality partner elsewhere.
You are not planning to divorce, since you still have feelings, but can you imagine living loveless in this cold atmosphere for the next 20+ years, day after day, just with him, since your son would be out? This long distance relationship will probably not work in the end, but regardless, it won't be his last affair. This is his drug now and maybe it always was, as somebody said above.
This is why you have to be prepared. He will drain you and one of you will escalate this situation. Just be ready, work on your mental strength. Because it can be even worse than this: losing your marriage and all your assets, while getting older, being left with literally nothing.