In the breeding scenario you proposed some puppies will be 4/5 lbs, but more will be 6 lbs. If both parents are 6 lbs, you are matching up those genes and effectively doubling your chances of producing more puppies who are 6 lbs than 4/5. All the dogs in his bloodline have an influence on the puppies' eventual size.
You will need to get the tests done as well as having him collected to evaluate his sperm quality. You will also need to learn how to handle him to get him to cover a female and you need to learn how to manage his "tools" before, during and afterwards. He will also be more keen to mark in your house thanks to the whole process.
Unless you have a dream of becoming a great breeder, don't put yourself and your boy through it. It's a lot of work for very little payback. Breeding is best left to the dedicated breeders, not pet owners who just want to enjoy their dogs.
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