Small bum

My boyfriend (of a year) has been commenting a lot lately on how I have no bum and how I need to do squats to get a bum. I told him I don’t like big bums and that I think my bum is fine. I probably had a bit more meat on it when I met him but I’ve since lost weight so the cellulite is a bit more noticeable. If I make a big deal out of it he tells me he was just kidding, but I know he’s not because he mentions my “no bum” every time I see him and has started calling me by a nickname of “‘no bum”.

Any suggestions on what to do? I’m happy with my body as is. If I tell him to stop calling me no bum he either jokes around or tells me I’m too sensitive.