My experience with coat growth is that the undercoat comes in first and the guard hairs grow out beyond it afterwards.
I think your dog probably just didn't get the heavy coat it should have inherited.
He's young for it but it might be alopecia
http://www.marvistavet.com/html/body_alopecia_x.html
Pom pups do go through a coat change stage but usually you see the soft undercoat coming in pushing out the silky puppy coat.
This might be due to a thyroid problem. You could have your vet check for that.
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