This link may give you more info:
http://www.yorkietalk.com/forums/gen...rt-thread.html
One excerpt stated: A puppy whose coat appears sparse or thin will, as an adult, have a thick, long coat of the correct fine silky, glossy texture. This coat’s growth is like a human’s hair, in that it is born with very little but by adulthood has an abundance of hair, the quantity having doubled and redoubled all during puppyhood (or childhood). Both colors will be totally clear.
Also, Yorkies coats are similar to human hair in that they do not shed, but hair will come out in the comb or brush when you groom him, which is shed hair it just doesn't fall out on it's own. Do you groom him at home? Maybe a volumizing shampoo would give him a thicker look, and in one thread i read where someone recommended Omega 3 supplements for their coat. I trim mine, not short but actually just the tips, and his coat appears thicker and is growing fast. Depending on how his hair lays, I can see his skin, but not after I comb through and straighten everything up. Damage to the skin and hair from scratching can also cause changes, but should return to normal in time. I couldn't really see a problem in the photos, I'll look again tomorrow. He's a cute little guy! I see you have other Yorkies. Are their coats thick, which would make his look thin by comparison? Also, I use alot of conditioner on mine which can make the hair lay really flat against the skin and I see more skin through his coat when this happens. I was told also that a Yorkie doesn't come into full coat until they are 5-7 years old. Can't think of anything else right now, so I'm off to bed! See you tomorrow!