If he's had all his shots by 14 weeks, they were probably started too soon which means he isn't fully protected. Puppies get antibodies from nursing which block vaccinations given before 9-10 weeks old. Unfortunately, breeders who sell their Yorkie puppies too soon (before 12 weeks) also give them their vaccinations too soon.
Puppies need the last set of vaccination at about 18 weeks for full protection. They are especially at risk for parvo if vaccinations are completed before then.
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