How can I explain to this guy I dont want him to f*ck with my emotions, be faithful and I'll give you a try?

So hes a huge flirt, hes had a few girlfriends before, madeout with a bunch, had for the first time, which he confessed to. He likes me, I know he does and he doesn't see why Im weary of him. I've been thinking seriously of just letting go, taking down my shield and letting him in, but I want him to know that I'm not here for games, dont be flirting with other girls behind my back, and such. Hes super sweet to me, which is why I like him. How can I get this message acrossed firmly? Thanks, guys. (Ah, ps. Sopres in highschool)