HELP ME

I’m 18 and my bf is 25, we started dating when I was 16 and he never really cares about our age gap but when we argue he always goes on about how I’m too young for him and when we hang out with my friends he always starts getting really insecure and starts saying to me that he feels like an old man next to me and starts doubting the relationship. I know he loves me he’d do anything for me but I don’t know how to make him feel better about the age gap. As I said he’s fine every other time besides when we argue and when we’re with my friends and I don’t know how to re assure him that our age is nothing and that it shouldn’t be a problem to him if we’re madly in love but he starts calling me a child and a two year old when we argue and it pisses me off so much I just don’t know how to get him to stop doubting it. Any suggestions?