Welcome! I am Dr. Altman, a licensed veterinarian and I am happy to answer your questions.
So sorry to hear Zack is losing so much hair!
There can be many causes for this condition in a dog his age
Primarily ruling out an endocrine issue such as hypothyroidism or low thyroid would be advised
This is a blood test by your veterinarian
Other causes include: stress, diet (either is not well rounded or a low quality), other metabolic causes such as liver, kidney, or gi issues
You can definitely try supplementing with a fish oil supplement that needs to have 750 mg EPA & DHA daily but this will take 2-4 weeks to see a significant improvement
A high quality organic diet with little or no by-products
But if this continues I would advise a veterinary examination and some basic bloodwork to rule out metabolic or endocrine causes since these are treatable to return his coat to a previously healthy state
Please let me know if you have any additional questions I can assist with today!