I had once mentioned a punch card system
Each provider is different. The course of action would differ from one provider to the next. Some of us out there discount, and others would scoff at the mere mention of one. I'd suggest you approach him with something like: "I have hired you frequently for quite some time, would you be open to adjusting your rates for longer sessions?"
One of my favorite quotes: "They will likely say No, but they COULD say Yes."