Sorry if I'm being clueless here, but for various reasons I will assume the dates you had with him included sex which I must also assume you found satisfactory if you now want to be exclusive. Maybe I'm wrong that to younger Redditors this goes without saying.

So far as I'm concerned, you are totally justified to ask him to be exclusive. The only way he could possibly decline is if he is some sort of player. I peeked at your profile, sorry, and found ample evidence there that you are a very attractive woman.

Unfortunately the kinds of dudes most likely to get matched on dating apps are that sort of player. Might be worth thinking about.