Assuming you have a written contract with him, look at the part where it says how/when you can terminate the contract or renegotiate the terms. That will be your starting point.
If he's a nice guy and likes/needs your work, he may be willing to renegotiate. But in the back of his mind he's going to be wondering if you are going to try to renegotiate again after a couple more projects. So you should prepare that any "guaranteed revenue" from him in the future might be 0.
Now, all this is based on typical business practices in the U.S./U.K. and other with similar business cultures. If you go to Korea or Japan, the real negotiation starts after the contract's been signed. :)