If you're that insecure, you really need to work on that before getting involved with anyone. You're not in a committed relationship with him .. So he can speak to who he chooses.
Get to know him better and you will be able to judge his character better.
If you're an adult who is experienced in dating, then I'd say that you are most likely overreacting or you've just had a lot of really bad dating experiences and I'm sorry for that.
However, if you're still a teenager and/or someone with limited dating experience, then I'd say that you're probably worrying over nothing however at the same time it's more acceptable and normal for you to have these feelings because you don't have enough experience to tell you otherwise.
Whichever one you are, if these feelings are unsettling you, then you should try talking it out with someone close to you just for you to get it all out and have someone who you trust reassure you. However, if this feeling is based on some evidence (such as this guy has a history of cheating for example), then you may want to keep your guard up and, if this continues for a long period of time, try talking to the guy about it. And when I say talking to the guy about it, I mean discussing how you feel about this situation rather than accusing/insulting him because that just never gets anywhere.
I dated a girl when I was in my early thirties. We would go places together and there were other men that she would talk to appropriately when we were out together.
We wound up being exclusive for a considerable amount of time. She once mentioned that I had just caused the others to disappear from her mind.
Confidence. It's all about getting to know one another and enjoying each other---it's not a contest.
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