13 day old kitten losing fur

Question
Hello and thank you for reading my question.  About 10 days ago we found a newborn kitten in our garage with its 4 dead littermates.  We received excellent help on this site and have been successfully bottle raising him on kitten milk replacer.  He seems very healthy, is gaining weight and growing.  Eyes are open and all seems good.

After each feeding he gets burped gently and gets a light massage.  He has a hot water bottle (IV bag) wrapped up in towels to keep him warm.  

My concern is this.  He seems to be losing his fur.  I don't think this is a result of too much rubbing/erosion, but wonder what it might be.  I know that human babies lose their newborn hair but have never seen this in a kitten.  His fur is very thin now and you can see the wrinkles of his skin through the thin fur.

Any help you can offer would be most appreciated.

Answer
Some kittens with poor nutrition can lose their hair, but I would like to also rule out a ringworm fungus as the cause.  Have this baby checked for ringworm by your vet. If it is not ringworm, then the hair may grow back when the kitten is weaned and starts kitten food.