My erection is not as firm when I am on bottom. Just do what ever works
Since you are confronting him about, did you ask if you masturbates and watchs porn at same time, most men who have ED issues with thier gf/wives/friends, never admit it.
Switching who is on top is nice for variety and often it is better for the female, but it just doesn't work for your partner. To me, it has nothing to do with you, your vagina, how you move, how fast you move, nor your attractiveness. As you've stated, he doesn't know why, but as he says, he just loses the feeling in his penis. Without that feeling, he is "high and dry".