I actually did this to a guy on a second date. (Not my smoothest move, I'll admit.) I genuinely liked him, but was still feeling things out (again, second date!), while at the same time, he was telling me how he'd already deleted his dating apps, wanted me to meet his closest friends, and was obvs. very invested already. I'd happened to plan a date with Dude B around the same time Dude A had asked me out (this is what happens when you go on a dating app binge), so I somewhat awkwardly mentioned it in conversation with Dude A. For me, it came out of wanting to be on the up-and-up with Dude A and not lead him on into thinking we were already on the exact same relationship page. Like I said, I liked him, but it was still just a second date to me, a time to test the waters, not declare our undying love for each other. His face fell, he shrugged it off, but I could tell he was hurt. But. Point is, it's likely this guy is feeling you out. Either he's trying to communicate that a) his feelings for you don't currently match your feelings for him and he wants to be honest about where he's at or b) he's interested in a relationship with you but isn't sure where you're at, so he wanted to see what you would do if he mentioned dating other people and if you would ask for exclusivity. Honestly, there's no real way to know unless you tell him you want to be exclusive and see what he does.