It sounds like he is looking to submit a proposal to someone, and he would need a contract to protect his position, a purchase order would only have the customers terms and conditions
I would bet money that someone who went to the trouble to have plans and specs drawn up had contracts drawn up as well.
It used to be real common for the specs to have the contractual details incorporated into them and the P.O. would just say that the contractor agrees to abide by the specs, and that covers it. Many times the P.O. comes with multiple sheets of T&Cs which by accepting the P.O. you are accepting the T&Cs.
I think it is not realistic for a contractor in this type of situation to expect that he is going to get the buyer to settle on a contract the EC has written.
But, YMMV.