Hey there,
Unfortunately this is very common on how folks do business these days. The only thing I would recommend to do is have a commitment from him on cost and completion date. He expects you to pay for his services, you can demand cost and completion date. Always make sure you know exactly what's being done, when and $$. A surprise will always be an unpleasant one. If you can't get a satisfying response, than he most likely will mess things up down the road. Than I would yank it from him. There are a lot of engine builders out there, he's not the only one. Hope that helps.