Poor old Mack truck needs help

We need help, My girlfriend has an 8 year old cat named Mack. He is THE biggest sweetheart, but he has some problems. When she got Mack, he was not properly taken care of and he was so skinny that his ribs were showing. Over the years, he's gotten so overweight that he cant see (or clean) his backside. There is another cat in the house who eats the same and has no problems. He sheds way too much, but the main problem is his feces is too soft and when he uses the box he has a mess. I don't want to get too graphic, but it gets stuck to his backside, and he drips all over the house, and also every time he sits he leaves spots and stains. This is disgusting and heartbreaking at the same time. The local vet said he doesn't know why, and offered no help or advise. We don't want to take him to the humane society, but we have no idea what to do. Is there anybody out there who knows what to do?

Has he been checked for diabetes? Soft stool & hugely overweight are two red flags. Does he drink a lot of water? This is the third most common red flag.
   
Diabetes is very common in cats (they tested human remedies on cats) & does come on in older cats & is easily treated with insulin shots. I did this years ago.
   
Between now & when you find out if this is the problem (we'll move on after we get an answer to the diabetes question):
   
Give him a Light diet.
   
Add cooked rice to his food; it helps stabilize the stool.
   
Give him vitamins (I like Vetri-Science Nu-Cat vitamins, reasonably priced at $13.00 for 240, 4 pills per day).
   
Clean his rear with a damp paper towel.
   
If it's not diabetes, it may be a food allergy. Get him tested for diabetes first.

Thank you very much, nobody else has still to this point offered any advise.
   
As to your question, no he hasn't been tested. We kind of ruled that out a while back, because the number one red flag that we knew of was the drinking a lot of water thing, which is not a problem for him. But now in light of this new information we are looking into getting the test done. We are taking your advise and giving him cooked rice and vitamins, and we will go from there.
   
Thanks again, you have no idea how much it means to us.
   
Sincerely,
   
Greg Horan
   
ghoran26@gmail.com