This was roughly Twinkle's weight at 4 months too! She's just over a year old now and weighs 12lb. The vet says she isn't overweight....just on the larger side for a yorkshire terrier, and I don't think my boy Wicket (nearly 6lb at 6months) will be far behind her! As someone has already said, a dog under 20lb in weight is very small for a dog.
Our other dog, kaddie, is almost 30kg at 6 months old, so
all other dogs seem tiny to me now!
Would love to see some pics of your pup....I bet he's adorable! xxx