You may want to get the police involved since his vehicle is now stuck at a shop that may be entirely closed. At least that way you'd know the proper legal grounds for getting it out since its basically locked in an almost abandoned facility. A few business days to a week, two weeks tops is enough. Several months of this is getting to be a serious issue.
"The music business is a cruel and shallow money trench, a long plastic hallway where thieves and pimps run free, and good men die like dogs. There's also a negative side"