Extremely Small Male Boxer Pup

Question
I am an owner of one female Boxer pup, aged 10 months.  Last week, I brought home our newest addition - an 8 week old male boxer pup.  He looks extremely tiny and weighs only 3 1/2 lbs at 8 weeks.  He has been checked by my vet - and her concern is his size.  I am very worried about his size and want to make sure I am providing him proper nutrition so that he grows.
I never had this problem with my female - as she was a big pup to being with.
The breeders I got the lil male from told me that he is small for his age due to the litter size (12 pups were born, 11 survived).  
Any advice on feeding and weight gain would be greatly appreciated.  I feel very lost and scared right now.
Thank You.

Answer
He should normally weigh 1 pound at birth and weigh approximately 10-14 lbs. now.

Feed him a natural puppy chow-- high protein, no corn and no corn gluten.

Feed him 6 times a day right now.

Add 1 cup whole, plain yogurt (no flavor) to each meal.

Continue this and add cheese for treats in between, and let him lick peanut butter (creamy only) off your finger as a treat also.

Add one raw egg mixed with the yogurt once a day.

He should gain weight within a few weeks.