Did he take care of your son during your weekend get away?
Perhaps broach this by asking what is his motivation for wanting to know this? And what if his worst nightmare came true. (if this is his nightmare). Perhaps he should leave it alone. Don't ask questions you might not want to hear the answers to.
But he sounds more mono in thinking and he needs to know this is life identifying thing and something you can't or wont shut off for him or anyone. (At least with his dick size and staying power
He might be making the mistake of viewing the relationship form how he feels and doesn't get,...... hopes .....wishes you felt exactly the same way...but sadly you don't.
This mind set difference needs to be discussed so you each don't waste anymore of each others time trying to change the other.