Why did my puppy lose his undercoat?

My American Eskimo Max was very fluffy when I got him. For some reason he shed his undercoat in early November. He has no fluff right now, I'm afraid that he is going to freeze this winter... We live in Pittsburgh. My Zoe is 14 and has never done this. Is this normal? Will his fluff grow back? Is there anything that I can do to help it grow back? I'm worried the my little buddy will be too cold!!!! Help!

P.s. He is not patchy, I'm not worried about thyroid or fleas