How can I tell my boyfriend about my kink?

Soo my boyfriend and I been tg 4 years. For a while now I started to have fantasies of an older guy and me… yk. My boyfriend and I are young (21 & 22), I constantly fantasize about it AND IDEK WHY. I love my boyfriend but I guess it just something I want to try?? Idk, should I bring this up to him? Just keep it to myself?