As I said before, the last thing I want is for him to feel rejected, especially since he has had enough of that in his life.
I've explained all this clearly and in detail to him. He thinks I am just overreacting and that this is all just harmless fun. I'm more concerned about his feelings than he is, and im worried that he may eventually feel hurt in some way,
Please don't take this the wrong way, but is it possible you are having some difficulty adjusting to the fact that this young man is a grown adult? I recall you mentioned he was 19+
As a parent, I've observed the difficulties some 'parents' have adjusting to their children no longer being 'children' and under their control. As a parent, I attempt to give my kids more autonomy and authority over time so we both have time to adjust before their magical 'adult' birth date arrives. And sometimes that means the kids make decisions I'd rather hey hadn't.
It sounds like you and the lad have some sort of father/son type bond in your mind. Perhaps even in the lad's mind.. except he wants some sexuality there too.(Not an uncommon attraction from what I see online. "father/son" "Big Brother / Little Brother")
My thoughts at this point, with the limited (and possibly incomplete) information at hand is to consider treating him as an adult, respect his decisions and take him at his word. Accept that while you may be 'older and wiser' and totally correct, he may need to make some less than optimal choices as part of living/learning.(And he IS a guy. Sex and feelings aren't necessarily connected.)
Treat him like you would any other adult.
The question then becomes, what do you want? What would you enjoy/be ok with? What are you not ok with? If the sexting bothers you, then put a stop to it. If not, then enjoy it. From the sidelines it seems harmless. Remember, sex is supposed to be fun. Can you laugh with him at the antics?
Is it time to become a peer rather than a supervisor?
What do you think? Am I anywhere near the mark?