I'd say something like this the next time you talk to him:
"So I've noticed you haven't seemed as interested in going out this past week as much as usual. I hope I don't make you feel like we have to go out all the time. I just wanted to make sure everything was okay?" Then after he responds assuming he says it's nothing, "So there aren't any other girls or anything?" If he says no, you say, "Good. So when we agreed to be exclusive, does that mean you can promise you won't be with anybody else unless we break things off first?" Hopefully, if everything goes well and he agrees, then you can let him know you didn't think he was that type - that you were just asking because you realized you'd never clarified that during your previous conversation. After that, you can change the subject. It shouldn't seem like a big deal to discuss something like that. Of course, if something comes up during the conversation or you don't feel like he's being honest, then you should have a longer talk. Communication is important for a good relationship.
“This planet has - or rather had - a problem, which was this: most of the people living on it were unhappy for pretty much of the time. Many solutions were suggested for this problem, but most of these were largely concerned with the movement of small green pieces of paper, which was odd because on the whole it wasn't the small green pieces of paper that were unhappy.” ― Douglas Adams, The Hitchhiker's Guide to the Galaxy