As Crystal said, shots given too soon after weaning don't take as the maternal antibodies interfere with them. Unfortunately, the shots he got at six weeks were probably worthless. He needs the last vaccination somewhere between 16-18 weeks.
I've heard too many stories of puppies getting parvo because their shots were given too early.
In one study of a cross section of different puppies the age at which they were able to respond to a vaccine and develop protection covered a wide period of time. At six weeks of age, 25% of the puppies could be immunized. At 9 weeks of age, 40% of the puppies were able to respond to the vaccine. The number increased to 60% by 16 weeks, and by 18 weeks of age, 95% of the puppies could be immunized. Parvovirus: Serious Diarrhea in Puppies & Dogs