I got Chico at 'almost' 6 weeks and he was on a mix of dry kibble w/water. He ate like a piggy - after about a week I had weaned him onto all dry puppy kibble. Take a stool sample into the vet asap. Have them check for worms.
Chico was extremely tiny (scary tiny) until he was about 5 months... way small and the parents (I was told) were very small also. I never saw the parents. I rescued him from a BYB.
He was always full of energy and healthy as could be, had a good temperament, loved people and other dogs. The vet wouldn't neuter him until he was slightly over 6 months because he was too tiny. He started putting on weight shortly after and ended up at 7.14 - 8.2 lbs. Love your puppy a lot, be sure to socialize him a lot because he is very young and needs his littermates - but if you do it right he'll end up just terrific.
Prince my first Poodle I also got at 'almost' 6 weeks... and he was great too. I realize that is way too young, but sometimes circumstances don't allow the puppy to remain with parents and littermates.
Good Luck to you!