If you want him to breed, you want him fully mature - 6 months or so. Bucks become fertile at about 3 to 4 months, so if you don't want him breeding, get him away from his mothers and sisters by 10 -12 weeks. Just because the buck is fertile doesn't mean he's ready to go; a young male that gets rough treatment from a dominant, aggressive doe may be put off breeding for a long time, maybe even life.