Thanks for answering my questions. I really appreciate it.
There are a couple things that come to mind with your big guy:
First, with the ongoing diarrhea for what sounds like a good part of his life, the first thing that springs to mind for be would be the possibility of something like Irritable Bowel Syndrome. This often presents with ongoing, long-term diarrhea and weight loss. Usually there is a test done to determine that the intestines are thickened, such as x-rays, ultrasounds or MRIs. However a first step could be to talk to your vet about getting him started on an antibiotic such as metronidazole or/and a steroid such as prednisone. These are commonly used to treat IBD and may help get things under control.
My second thought (and less likely option) is the possibility of an issue called protein losing enteropathy. If it's determined that he has this, they may decide to try using ACE (angiotensin converting enzyme) inhibitors to see if they can lower the amount of protein that he's losing.
Since these illnesses (the IBD and PLE) are not common, there are many general practice vets who aren't familiar with them or how to treat them adequately. This is why I suggest getting a referral to a veterinary internist (they can sometimes be found at veterinary teching hospitals).
With both IBD and PLE, the vet will often suggest a special diet. I know that you've recently switched to a new one, but your vet (or the specialist) may want to look into feeding a no wheat (also called gluten free) diet. Looking into a novel protein such as fish, duck or venison may also work better than the more common options of chicken or beef.
If everything comes up negative, or if your vet doesn't think that it could be IBD or PLE, then it might be time for a referral to a veterinary internist. These are the folks who deal with unusual internal problems day to day and may have a better idea of what could be going on with him.
I hope this helps.