No winter coat

Question
We have a 6 yo Corgy/Chow mix that up until this year has always produced a heavy undercoat.  He is outside except at night and weekends off and on.  This winter he is shedding like he does in spring/summer (clumps of inner fur) to the point where he pretty much is down to his outer layer.  We live in Denver so of course we are concerned.  Advice?

Answer
Some of the heavy coated breeds will get this so called Alopecia-X.  The undercoat just won't grow.  Some of these dogs are hypothyroid, while others will respond to melatonin.  Check with your vet.  http://www.marvistavet.com/html/body_alopecia_x.html