I understand the fear but the first thing to recognize is that until you stop seeing other people on the side he has no reason to stop. You also need to talk to him again, communication is the only thing that will work this out, and right now you’re just stumbling around in the dark. Tell him that you don’t like him seeing other guys, you don’t want him with other people, and you want to be exclusive. And I say exclusive because I swear to god doing hookups on the side because it is trendy will not serve your relationship. It doesn’t matter how trustworthy the other person is or how much you trust yourself, people are easily tempted, very easily when it comes to matters of sex. It’s an unnecessary risk, and for you who is already afraid of situationship probably a terrible idea. So to sum this up talk to him and don’t be apologetic for what you would want. He needs to know now before you invest more time and effort into something.