There is no legal obligation on him to sign the work work order: his obligation is to pay for the work which you did. If he did not pay, you could could sue him for the money; if he is paying, though, there are no grounds for you to do anything.In the future, if uncomfortable proceeding without a work order--and if you have to sue for nonpayment, the work order would definitely help, to prove the work was authorized; it is a good thing to have--simply refuse to do *anything* until and unless the work order is signed.