While I am not a docotr DVT may be a problem but there are usually no symptoms with that. We all have times when are knees give out while walking but most of us catch ourselves before we fall. Your Brother In Law problems may be do to the fact that he is severly overweight if he could get his weight down to around 190-210 depending on his frame he would see a dramatic change in symptoms.
You also need to remember that just because he doesn't fit the text book description of gout doesn't mean that isn't what he has. I have a disease but only have about 4 of the 20+ symptoms of it.
If you are doubting if he has gout have him see a new rhemuatologist for a second opinion. You should also make sure he is following his diet and not cheating on it.
Does he also have fatigue & other signs of CFS, FM or CFIDS?
"Gout is a type of arthritis that results from too much uric acid in the body. Rather than being flushed by the kidneys and through the urine, as it normally is, it forms crystals and deposits in the joints. These deposits give rise to inflammation of the joints, causing pain, swelling, redness and tenderness of the area."
More info
http://www.arthritis.ca/open%20forum/boards/gout/default.asp?s=1
My question ... does he have some exposures to 2-butoxyethanol?
Could be
He should study up on this chemical