HELP how do I tell him....?

We arent together officially, we havent even talked about what we are. We have talked every day for about a month and a half, work together every day, hung out, made out, and have told each other that we like each other but.....last night I was drinking with my friends and I made out with one of my good friends. I feel like I should tell him but I dont know how to say it. How do i tell him I hooked up with another guy in the least harsh way possible?