I think perhaps you should tell him that you can't start on the last two projects until ALL of the payment/compensation/reimbursement has been taken care of for all of the previous jobs. You simply can't wait any longer to get this straightened out. And since that's what you wanted to talk to him about....well, just let you know when he's free to talk, and then you can make arrangements for the other two jobs after that.
The other two jobs are your bargaining chip.
Then, like a PP suggested, say that you'll need advance payment for the other two jobs because you simply can't afford to wait, chase him around, wait for him to be in a good mood, or argue about valid payment any longer. Be blunt, but professional. Because I'm pretty sure that he would never have been able to get away with such unprofessional behavior with anyone else. Which means that if he wants the work done, he's going to have to start acting professionally. Why not start with you?