When will male cairn terrier be ready for breeding?

There is more to breeding a dog then male+female = cute, healthy puppies. Your dog needs to be mature, I wouldn't suggest breeding him until he's fully mature. Also you need to make sure he's in top health. That mean having him fully vet checked for any potential problems such as hip issues or genetic disorders. He should also have a desirable temperament. Temperament is genetic as anything else! Also, if you plan on breeding him and raising the puppies yourself, you need to make sure you can afford vet check ups, shots, and have the time and space to deal with pups. Do your research! Rash breeding or not spaying and neutering creates puppies that eventually fill up animal shelters.

Ask yourself this. Why do you want to breed your dog? Have you found homes for the puppies that he will sire? If not, are you going to keep the puppies?

There are so many unwanted pets out there, our shelters are full of them, why add more?

Leave the breeding up to Expert breeders. Just enjoy your dog.