Whenever I invite someone to a concert I discuss with them immediately who will be responsible for ticket purchasing. Usually it's me. For a seated concert tickets should be bought together, but if its standing room only it doesn't matter if tickets are bought together or not.
How did he ask though? Did he go out of his way to ask if you wanted to go OR was he discussing the concert when you expressed interest? If he went out of his way to ask you he doesn't have particularly good people skills.
'People are never perfect but love can be. People waste time looking for the perfect lover rather than creating the perfect love' - Princess Leigh-Cheri from Still Life With Woodpecker.