Whatever you do now, I would make sure that you don't give him the idea that you are interested again. Don't go back to him if things don't work out with this other guy, don't complain about your love life to him, etc.

Like other posters suggested, it is best to be up front and frank with him. Tell him that you are not interested in him romantically. Tell him that even your basic friendship makes you uncomfortable at the moment.

I have found that it is sometimes best in these kinds of situations where the line between friendship and romance is crossed and then needs to be uncrossed is to cut off all contact completely, at least for a while. Anything else may give him the wrong idea or false hope.