Most Helpful Guy
If there was a video/pics and he's seen it too, it would have jogged his memory and he would have solved the puzzle of what happened at the concert. Also his friends would remember some parts of it so i am sure they have spoken about it amongst themselves if he honestly couldn't remember (as drunk people do inc myself)- that would have given him more pieces to add to that day, this would have jogged his memory to remember at least one moment of that time.
I find it incredibly hard to believe him, but he may still be innocent. Don't bother asking his friends, and if you do, don't believe them.
I'm sorry to say this; but as a guy i can say if he really was committed to the relationship, then he wouldn't have tried it on with other girls, even if his boys were to tell him to make movements.
Girls should really stop believing guys over everything. Don't be afraid of the relationship breaking down if it does, because the further you go on it will only hurt more. From now on keep a very close eye on him without him knowing. And another which i suggest you do if you're brave enough is to text him in the next 10days on a different sim card and try sexting until you arrange a meet up. When the day that he needs to meet the fictional 'girl 'comes, see what his excuses are as to why he can't meet you. Then in person confront your boyfriend and ask what he's up to- again to see what he does.*Make sure you're able to say where you met. Best to know when and where he was drunk so you don't mess up the trap. I hope this helps, all the best.