I would just tell him, were it me. I do think it was wrong of him to try to kiss you if he is in a relationship, and I think it was very wrong that you had to tell him no and push him off you more than once. That, to me, speaks to a lack of respect for you.

You aren't leading him on or getting in the middle of his relationship...at least on your own, in the case of the latter. You aren't doing anything of the sort to lead him on IMO. As far as getting in the middle of his relationship, if he puts you there by doing this, that's nothing I would blame you for. That's his fault.

If I were you, I would tell him that you are not interested in being anything more than friends with him, and tell him that if he acts the way he did last time you two met, you aren't interested period. That's me, anyway.

I hope I was of some help. Take care.