Any shop worth anything would not want to put a new coat of paint over unknowns. It takes a lowlevel shop person less than a day to take the whole truck down to bare metal with the proper sanding discs and a variable speed polisher/sander.
Now he can finally assess how much metal repair the truck needs. Only then should he be thinking about fillers. He can then use any brand primer and finish coat he wants.
Stop the work. Ask him how much for the work he did so far. Come to an agreement. (He has you by the shorthairs). Then find a real shop.
