what do I say to this guy

so this guy and I used to talk for a little bit, but I wasn’t interested so I kinda ghosted (oops). we stayed friends on snapchat and such and in the meantime I found someone and he is now my boyfriend. anyway, this guy has a friend who is dating my best friend and my best friend tells him all about my relationship with my bf (idk why). me and my bf went through a rough patch and broke up for a while and tbh our relationship seems kind of unstable now but we both are working on things. I think my best friends boyfriend told the guy I used to talk to that me and my bf are having problems and now he sees it as a vulnerable time for me and he’s trying to make his move. he texts me all the time and starts conversations and he’s on vacation right now and told me that I should have came with him and he added a winky face.... at first I thought it was casual conversation as just friends but I see he’s trying to flirt now.

I don’t want to be one of those girls who is like “uH i HaVE a bOyFrIEnD” but I just want him to know so he stops flirting and we can just be friends.

should I say something like “not sure how my boyfriend would feel about that” or “I don’t want to be rude, but I have a boyfriend now”. are those things rude or too harsh? :/ help!