[Quiet Man Cometh]

Forget the Mona Lisa. Go get a full scale replica of the statue of David and drop it on him. ...

Okay, maybe that's a bit much, but I agree that you should make some effort to either get the art done or get your money back. If he decides to gripe and not want to talk to you anymore, remind him that you paid him for a service which he is now obliged to provide, or he can return the money. The remark about AG Bell makes him sound fairly imature and definately unprofessional about his art commissions, even if it's only a casual thing for extra cash.