If the changing area was designated women only I wouldn't have taken him in.
If it were communal I wouldn't have taken him in.
You wouldn't have seen him because he was in a cubicle. He tried on, he got dressed, he came out.
Everyone had their own private space because they were cubicles. If they want more private then perhaps they shouldn't go out in public where a fully clothed boy may see them whilst they are fully clothed 
As for his sensibities, I didn't take the poor lad through the lingerie dept, we detoured round it and i didn't take him to this changing rooms.
If he ever gets in trouble for inappropriately physical contact at school I'll be worried as he goes to a boys school 
Incidentally, none of the women who were there when we were batted an eyelid. one, clearly a mother, said how good I was getting him to try stuff on.
I didn't parade him about... We went in, he went in the first cubicle off the corridor and came out when he was done. I sat outside his cubicle in the corridor. Nothing to make it women only at all, no mannequins with underwear on or anything.