suggest
1) You file liens on all work you have not been paid for in full.
2) From now on give tour extra/change order fair price only to him, Tell him he is the general and cutomer relations are his job. If he still wants you to give the customer the price give them the same price you gave him. When he discovers that no markup for him is in your price, you will have his attention.
3) Find another general to work with and commit more of you time to the hopefully more honest guy.
4) Raise your price to the current guy with a larger down payment asked for up front.
5) Be sure you charge intrest on late bills.
6) Tell him no more work will be started until all past due bill are caught up.
This will hopefully allow you to walk away slowly enough to have a chance to be paid. If he accepts these changes, and gets current on your bills, you may have a higher profit customer. I personally doubt he will accept the changes so be prepared to go to court.
