I met this guy from a dating app so it's natural I guess for him to assume that there's a chance. I really do like him as a friend though and I want to get to know him more, how do I tell him that i'm currently seeing someone and I just want to be friends? We spoke before I started seeing someone by the way, I'm not intentionally trying to hurt anybody.
Make sure you tell him soon the longer it goes on the more it'll hurt when he hears it. Just be clear with what you want, that you're dating someone but you enjoy talking to him and want to be friends.
As has already been said, just tell him. And you will have to accept that he will probably not want to be your friend. Guys don't think that way. You friendzoning him right off the bat will be seen as a rejection clear and simple. Hopefully the guy will move on from that.
Maybe just ask him if it's okay to be just friends... being open and saying you're dating another person but you like him and would love to stay in touch, is best. Otherwise he may be hoping things, and there's a good chance he'll find it okay if he likes you too.