Cheating

Me and my boyfriend have been going out for almost 7 months now and he's been the best boyfriend I could ask for throughout these 7 months, but last week I was walked home by a friend from school and I'm guessing he liked me.... He kissed me after walking me home and I didn't mean to kiss back, but you know it's unstoppable... When someone kisses you, you tend to kiss back but I really do like my boyfriend and we are honest with each other... How would I tell him without him breaking up with me???