Is it possible he is going through the "puppy uglies"?
Do yorkies go through them? I know Pomeranians do and Adorable fluffballs as pups, homely little scruffies from about 3-7 months and then usually by 1 year they look fantastic.... Just another thought on why he might be a little thin...
Here are some poms going through the 'uglies'....
Puppy Uglies Pom Haven Pomeranians