Most pups start teething around 16 weeks, and the coat changes around that same age(starting with the spine/fur becomes shiny there). You won't really know the pups true age until that begins. If you see it happening late, you'll know the breeder probably fibbed on the whelp-date.
If you over feed he'll get runny poop too, so feed 3 meals a day and around the same age(16 weeks) is when I go down to two mealtimes a day with a midnight snack to keep the gut from developing too much digestive juice.
If you have to vaccinate for rabies, make sure you do it 2 weeks separate from the other jabs. There are many threads about vaccinations/diet and other young puppy stuff in these threads. Happy reading!
Please take some pics, puppy photo's are mandatory!!