Poor doggy! Like Lucky said, 6 weeks is so young. Not only will it affect him now, but later on he might end up with a bunch of sociability issues... Is it at all possible to bring him back for at least another couple of weeks?
Wellness is good, but might be hard on his belly if he's so young. If you have Merrick dog food in your area, I would look into getting the canned food for a bit, just until he gets stronger.
Merrick Puppy Plate canned