The hair loss could be simply that he's 'blowing coat' right now, which is basically a major shed that most dogs experience during the change of seasons from cold to warm. The fact that it's gray in color could indicate that it's his undercoat that he's losing and that's normal.
However, severe hair loss that results in bald patches, or if his skin seems itchy, has a rash or looks red, it could be that he's suffering from canine allergies, or has a parasitic condition such as Mange, or Roundworm.
I'd strongly recommend that you take your dog to your veterinarian for a check up to make sure that he's not got any underlying health issues causing his symptoms. It's always easier to 'fix' something in the early stages than to 'wait and see' and end up with a more serious problem.
Best of luck with your dog, hope he's doing better soon.
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