I don't understand your description of the situation. Both YOU and YOUR SON should have been moved together out of the exit row. Not just your son, which is what you imply (my bolding above). I find it very very hard to believe that two able-bodied adults sitting together on a full plane (either a couple or two unrelated singles) could not be found to willingly switch seats with you and son. It was a problem of the airline's making and a problem they are required to fix before departure. Which apparently the FA's begrudgingly did, but it shouldn't have gotten to the point where they had to take responsibility for the fix.

I get the impression that once assigned the exit row inadvertently, you didn't exactly want to move out of it, and didn't make much of a stink at the ticket/check-in counter nor at the gate, where it could have been resolved prior to the start of boarding. Perhaps you were hoping nobody would notice and you and son would get to keep the exit row? Then when called on it, tried to just have just your son moved, and other unrelated adults rightfully refused to take responsibility for him sitting alone? Is that what really happened?