Well the fact that you guys are friends with benefits means that you aren't in a relationship and it shouldn't matter to you who else he decides to sleep with and it shouldn't matter to him who else you decide to sleep with. That's the whole thing about friends with benefits. You guys sleep with each other and you do this more than once to regularly but you both still have the freedom to sleep and date whoever you want with no strings attached. So if you are starting to get worried about who he's sleeping with or vice versa, then you two need to talk about your current relationship because worrying about all the sexual things that the other partner is doing isn't how friends with benefits work. That's why for now don't worry about what he's doing and do whatever you want without worrying what he has to say because the two of you aren't officially together and there is no need for any worrying going on. Just remember that if anything starts to change it means that your relationship isn't a friends with benefits anymore.