I had the same situation w/a girlfriend and I am female.  I told her the truth that I wasn't interested in being that way or even experimenting. Just be honest and tactful and tell the truth about how you feel....You care about him but are not interested in those type of relationships.  You just want to remain friends but that if that were going to be a problem for him then you will understand if he can't be friends with you. It may hurt him too much to be friends and see you but not 'be' with you.  So be understanding and hope he will be too.