insecure or valid reaction

So my boyfriend told me he'd be "happier" with me if I had a fatter ass. I kindly explained to him that no woman can just grow big ass ass n hips out of nowhere that it's genetics. And even if I did do squats he wouldn't  see that huge of a difference. Is it ok that he says he would be "happier" with me if my ass was fatter? I didn't take it to well because when I was younger I used to be SUPER insecure about my lower body & I feel like u should b happy with your SO no matter their body part sizes , I wouldn't change anything about him for the world and it made me feel really insecure again . What should I do . 
** he just told me I should be lucky he's not cheating on me with a bitch with a fat ass