Easy, at the time he refuses to sign anything, just say with a smile "No problem then, but I will have to mention that you refused to sign it while I complete my voyage report/trip record"!!! Once you have covered your ass then your covered!
OR
You could whip out your mobile and just say you will give Ops a quick ring to make sure its ok to go without the paperwork signed because the captain refuses to sign it without valid reason! That usually shuts them up! Fortunately you will only find a handfull of idiots like that!