My bf of 2yrs is always staring and smiling at other girls, and it makes me jealous! what can I do!!?

Our relationship is awesome. But when we are in public or driving, he would face towards any girl around and will be looking at her or he'll glance at her everytime he looks around. He knows I dont like that. I dont mind if he checks out girls but not just smiling and staring at other girls. To me its like he's telling them he likes them and they may think he is single. We always smile and look at each other. It just bothers me and I keep thinking of it or when will it happen again. I'm like obsessed with it and I just imagine he's trying to get into another relationship and leave me. But thats not it, He's so in love with me, I can tell. I am beautiful! and our everything is good. I feel with this jealousy problem, I may ruin my relationship with him. I some times think he knows that I'm watching thats why he'll do a ninja of ogling and smile stareing at other womens when I glance the other way or something. I also think he does it cause he knows I'm jealous when he stares and smiles at other women. Theres no doubt, he is good looking. What if I tell him I dont care and he can stare and smile at other women if he wants to, but if he does, I think I would get upset. I just dont know what to do?