Unfortunately, he's up the creek as far as any way to verify any of his claims. No receipt, and no proof of ownership of tank serial #XXXXX pretty much seals it.
I'm sure the kid made a dumb mistake, and the manager just doesn't want to deal with it. If the kid were in on it he never would've admitted to ever touching your tank, let alone giving it to someone else. The manager's attitude is it's your fault for leaving the tank and not getting a receipt.
In short it wasn't premeditated fraud, or even intentional fraud. Stealing 100lb propane tanks is not exactly a big money-making scheme.