What age is a male pit bull fertile?

At 10months, but it is not good for the dog to sire a litter till 13-15months old.
Letting him grow out of puppy stage first. It is wise to get him brucellosis-tested, negative, before he sires a litter, because if he's infected the disease or becomes infected through the dogs he's bred to, he can die, and so can the other dog.
(puppy stage is up till 14months for some dogs, 18months for others)