Hi,
So you say you actually want a relationship with him? Then I think you should tell him you really enjoyed the time together and would actually like to date him if he didn’t have a gf. And since he does you think it’s unfair to yourself to keep in contact with him so you are not going to. I think there is no harm in saying that, it’s honest and shows no hard feelings.
Though I run this site, it is not mine. It's ours. It's not about me. It's about us. Your stories and your wisdom are just as meaningful as mine. 