I'm going to a concert without my boyfriend
Is there a reason why you would go to a concert without your boyfriend?
He doesn't like the band, so I'll go with my friend
That's cool. It's perfectly fine to not go to events with your boyfriend sometimes. Time away from your boyfriend can sometimes make your relationship stronger.
I'm pretty sure he'll be fine. I'm going to a really big concert.
It sounds like you and your friend will have a great time at the concert. I think it's wonderful that you and your boyfriend have a good relationship but still maintain a sense of self and independence.