That rapid of weight loss sounds like some form of cancer, or possibly some form of toxin. Serious question, is he married or does he have someone who would benefit from slowly poisoning him? If he has seen an oncologist then I am fairly certain that he was screened for cancer, so my bet is on something environmental.
There are plenty of toxins that would kill someone slowly but are extremely difficult to detect unless they are specifically looking for them.
The cancer/leukemia has been a concern from day one but all the tests came back negative. He lives alone with a couple of dogs, we think thats how he contracted Lyme Disease. You're right about outside poisons, they almost have to search for each one individually. I don't think anyone has any motive for something like that, he was actually taking care of his parents until this happened.
If he is living on well water I would stop that immediately.
Is he taking any daily vitamins?
I'm not sure about the vitamins, but the well water comment is intriguing. That could hold some possibilities...