Contractually you don't have a leg to stand on, no surprise as you don't have a contract, he would just maintain the debt has been fully paid...in a "your word against his" situation the only winner is the lawyers on either side picking up the fee/ fees.
As they say, you slide further on bpoo than gravel: I think your best option is Robert Phil's suggestion of the carrot. I wouldn't send a letter as its pointless, unless you like writing letters. I would approach him as an old friend. Tell him you're still in contact with a great number of your old customers and they are really happy.......hence would he like to discuss taking over the remainder of your round?
I would exaggerate the figures......but not wildly, then gently suggest he stepped up payment of the existing loan as obviously you cant really be expected to hold more of his debt.....you could also feign an injury, make your situation more plausible.....you want out and only have eighteen months left in the biz due to health....
talk it up....maybe even throw in a few commercial contacts your happy to introduce....room for growth etc. Just my opinion.