KahaneWasRight,
Certainly this should be checked out, but whatever the cause it is healthier for him to be at his current weight than 260, especially as a male, since men primarily store fat around the abdomen, which is the most dangerous place to store it.
White Israelite has been through a whole lot lately; I am not an MD, but a lot of this probably has to do with both his general state of mind and the medications he's been on, neither of which are good for the body. I think those should be ruled out before assuming a "nightmare" scenario. We also do not know exactly how much he is eating and what that means without a caloric count. +-2000 calories/day is what is needed to maintain weight for an average male under average circumstances. He could be eating a lot of very low-calorie food, or eating a lot but rarely, or eating frequently but very, very little. We really need more information here.
The absolute worst case scenario, advanced metastatic cancer, would more than likely be causing more symptoms than fatigue and weight loss--like severe pain and obvious organ failure. Same with a serious glandular disease like Addison's--his heart would be always racing, he would be hot, and very excited and uncomfortable. Celiac disease and other digestive disorders would probably result in some sort of diarrhea and vomiting. I suppose he could have a bad case of tapeworms, but unless he has recently been to a third-world country, that is unlikely.
Hyper- or hypothyroidism, if there is not a thyroid tumor involved, is among the least severe of hormonal imbalances to treat. It involves simple bloodwork and corresponding amounts of the corrective medication, which I understand is among the cheapest of all synthetic hormones.
As for the milk, to each his own in terms of taste, but I don't believe that G-d would create unhealthy basic ingredients for us to eat (whole milk). Saturated, or animal, fat has gotten a completely unwarranted bad rap over the years thanks to heavy lobbying by the soybean industry and its flunkies in the "medical" establishment" on behalf of the various synthetic lipid substances it has promoted. There is nothing inherently bad or unhealthy about heavy animal fats--they are just very calorie-dense. They are not somehow magically worse for your heart than other lipids.