kissing another boy

me and my now boyfriend have been together for 8 months but only official for about a month. he moved away just for the summer and we weren’t official when he moved away because he said he didn’t want me to feel tied down all summer however we were still very much together and talked every day and were “exclusive” without the title. i got drunk on my birthday when we weren’t official and he was gone and kissed another boy and all my friends saw. my now boyfriend asked if i have done anything with anyone since he’s been gone and i told him no since i know the kiss meant nothing but i’m scared because all my friends saw and i don’t want it to accidentally come out one day that i lied to him. i don’t know what to do??? should i just leave it as is since we weren’t official or should i tell him because i don’t want him finding out from one of my friends? i don’t think they would tell him but things happen when you get drunk and don’t want it to slip out. help!!!