Hello, my name is***** and I have over 20 years of experience as a veterinarian. I am very sorry to hear about Bo's fecal incontinence.
There are a few things that may be causing his incontinence.
The first would be spinal arthritis which can cause inflammation of the spinal nerves that control urine and stool continence. These dogs have obvious spinal arthritis on radiographs and are painful upon examination. They may respond to anti-inflammatories (steroids or nonsteroidals like Deramaxx or Rimadyl), pain medications like Tramadol as well as omega 3's and glucosamine/chondroitin supplements like Dasuquin or Cosequin DS. You might discuss trying a nonsteroidal anti-inflammatory prescription with your veterinarian.
The second condition that could cause incontinence would be a degenerative myelopathy. This is a degenerative condition affecting the nerves of the spinal cord, it starts in the back legs and ascends eventually affecting nerves to the bladder and rectum which leads to loss of continence. These dogs aren't painful. They are uncoordinated in the rear because they have lost the ability to feel where their rear legs are and place them properly. They become incontinent because they can not feel the stool building up and because their anal sphincter becomes loose. Unfortunately we do not have any good therapies for this condition. Radiographs look normal and diagnosis is by a blood test to look for genetic markers of the disease.
The other disease process would be lumbosacral stenosis and instability. This is an instability between the sacrum (pelvis) and the lumbar spine. It leads to inflammation and scar tissue and places pressure on the spinal cord leading to loss of function in his rear legs and stool and urine incontinence. These dogs are painful and should respond at least somewhat to steroids or anti-inflammatories. Diagnosis is by an MRI of the area. Treatment is surgery but once fecal and urine incontinence occur we cannot always reverse the damage done to nerves. Definitely worth trying though.
Unusual causes can also be a spinal cord tumor or a mass of the vertebrae or supporting tissues.
If he is an intact male, was ever used for breeding and is now running a fever Brucellosis is a possible cause as well.
A bacterial infection of the bones of the spine that spreads from a urinary tract infection or a puncture wound is another possibility, but those dogs are running a fever and are extremely painful and feel horrible, and it doesn't sound like your fellow feels poorly, just uncomfortable and weak.
In order to help Bo we really need to know why he is the way he is. An examination and radiographs of his spine are a great place to start. If he shows signs of loss of feeling then testing for myelopathy would be recommended.
At that point your veterinarian can prescribe an anti-inflammatory and pain medication as needed.
In the meantime it may help to increase the amount of fiber in his food to help him feel and be more aware of his stools. You can add 2 to 3 tablespoons of canned pumpkin or Metamucil to his meals to increase fiber. Make sure to get him outdoors frequently to pass stools as well so his colon is less likely to overfill causing stools to just drop out.
It may also help to use a glucosamine/chondroitin product (examples are Dasuquin or Cosequin) and an omega 3 fatty acid (like 3V Caps or Derm Caps). These work synergistically and improve cartilage health and joint fluid quality and quantity as well as reducing inflammation. They can take several weeks to see full improvement but some dogs do very well with them added. They are available over the counter. I think that the product you are using contains glucosamine/chondroitin, but I am not sure it contains omega 3's, or if it does if they are in sufficient quantities.
Another option is a product called Duralactin. This is an anti-inflammatory product derived from milk proteins and it also has omega 3 fatty acids incorporated into it which can be very helpful. See this link for further information: http://www.duralactin.com/products_canine.htm
Other alternative therapies such as cold laser, acupuncture or chiropractic may be of help as well is his problem is arthritis related.
Let me know if you have any further questions.