Any suggestions?

Keep good records, document every thing, phone calls, emails, when you
paid money ect. If this repair was mostly cosmetic (labor) and little
material there is no reason for a shop to have you prepay, that takes
most of the motivation out of the desire to finish the job or in your
case start. Get a hold or you region director and /or the local FBO
at the airport that the repair shop is at and see if they will gently
lean on them (him?) If you can't get satisfaction be prepaired to go
to small claims court.