I am sorry to hear your pup is having some troubles.
Based on what you are describing, I am most worried about unilateral (one sided) facial nerve paralysis, where the nerve that innervates the muscles of the face is not functioning normally. Since they can't keep their facial muscles tight, they often drool out of the affected side of their mouth.
Here is a picture of what unilateral facial nerve paralysis can look like:
There are many possible causes of unilateral (one sided) facial paralysis, including:
- idiopathic facial nerve paralysis (dysfunction of the nerve that innervates the muscles of the face for an unknown reason)
- middle or inner ear infection
- traumatic injury (very unlikely if there isn't a history of trauma)
- endocrine diseases such as Cushing's disease and hypothyroidism
- immune mediated diseases such as myasthenia gravis (usually not one sided though)
- bolulism and tick paralysis (again, not usually one sided)
- cancerous process in the brain and/or along the nerves or nerve sheaths exiting the brain
Since there are so many possible causes, ranging from minor (idiopathic) to very serious (myasthenia gravis, botulism, and cancer), I would recommend having your dog evaluated by your veterinarian as soon as possible, especially if there are any changes in his appetite or water consumption due to nerve dysfucntion.
Tests that I would recommend include a complete physical and neurologic examination, complete bloodwork, thorough ear exam, and a Michigan State University complete thyroid panel. In addition, since the facial nerve is important in tear production, I also recommend a Schirmer tear test to evaluate tear production and insure that he doesn't need lubrication in the eye on the affected side.
I hope this information is helpful to you and that you are able to know what is going on with your guy soon.