I'm pretty sure you're not suspecting this is a communicable illness and you're probably right. I don't really think this is related to his intestinal issues either, though of course that possibility exists until ruled out. What I do suspect is that this is thyroid related.
Thyroid disease is always a possibility in an ‘off' acting animal from the age of 5 or 6 years and up. Not to say that it cannot present in younger animals, but it would be unusual.
, it's the number one endocrine disorder in dogs.
While T3/T4 blood panels are very helpful in diagnosis, especially taken together with the owners evaluation of symptoms, it's still not a sure thing.
Weight gain, dull coat, skin lesions/sores, infections, no energy and either an increase in eating or marked decrease may present as evidence.
The dog's bark may change, sounding hoarse and sometimes you can even feel a lump or swelling in the lower throat area at the neck.
Even if the tests are negative, if the symptoms are present a vet may prescribe thyroid meds and then wait to see if the symptoms alleviate. If they do, well, it's thyroid disease. If not, the search is on for what else it could be.
Here is more about this http://www.canismajor.com/dog/thyroid.html
Even if a thyroid test was previously done, there are many instances of false ‘negatives' (the level readings are misleading). For this reason a T3 and T4 panel should be done for extra sensitivity and perhaps done repeatedly for a while until some other cause is determined and treated.
You can find out more about this here http://www.canismajor.com/dog/thyroid.html
T4 hormone (Levothyroxine) such as Soloxine by Daniels and Synthroid by Flint are often better options than other Rx's on the market. These have a higher safety margin and tend to be more effective. T4 must first be converted by the body into T3, whereas Rx's of T3 hormones are intercellular (toxic potential).
In some cases both T3 and T4 therapy are used (and necessary) depending on the ability to convert the T4.
If you have a teaching school anywhere near you - it might be worth trying to have your companion seen there. These are fresh eyes that are looking for everything and focused on a solution with the most recent and updated information possible. Ask your local vet's, Humane Society, etc. for guidance to such a facility or options about specialist consultations.