If he's small, he may be considered to be too small to be suitable to breed from.

The best thing you can do if you want to learn about breeding and standing dogs at stud is to get involved with the breed. You will find the Yorkshire Terrier breed clubs listed on the Kennel Club website. They will have shows/seminars/meetings where you will get the best advice about a) your dog and his potential as a stud and b) breed specific advice. You will have time to learn as the current financial crisis means that responsible people are cutting back or delaying any breeding for now.

IMO using a dog at stud requires a lot more experience than having a litter, so should really only be done by experienced breeders.

If you just want your boy to sire a litter and leave it at that, then I would advise against it. If he is a pet, it could cause changes in character and temperament that you will regret.