Molly was 12 weeks, but was still tiny, tiny.....11 oz. She's almost 4 lbs. now and will be 2 years old in august. We actually considered having her stay longer than the 12 weeks, but the breeder was close-by and I had Molly with me 24/7. She was never alone and I understood everything involved with caring for a tiny little one. Since your little boy is a good healthy weight, perhaps your breeder knows best. You're going to be a great Yorkie mommy!
Congratulations!