You won't want to hear this, but I think you should back off. I'm not denying that there is probably a great sexual attraction between you, but let's face the reality. He's not going to want to have a long-term serious relationship with you which ends in marriage. Sooner or later he's going to want the serious things in life like having children and I very much doubt whether he's going to want you to be the mother of his child, or whether he will want to be the step father of your 26 year old son.
Your son is going to be mortified, humiliated and embarrassed when he finds out. Yes, OK, you're free to do what you like and your son is not your keeper, but he is more likely to be around in twenty years time and this other guy isn't. I wonder whether it's worth destroying your credibility in your son's eyes just for a fling. You're his mother for goodness sake - somebody he should be able to respect and look up to. It must be enormously flattering to have the attention of somebody who is twenty years younger than you, but try and see it for what it is and don't let the excitement of it go to your head.