Although there are some slight variations depending on breed, most male cats reach sexual maturity at around five to seven moths of age. Castration or spaying of a cat can be carried out as early as seven weeks if desired.
In general, male cats mature physically, between six and twelve months, and breed later than female cats. The actual time a Tom first breeds depends on being psychologically mature.
When you do breed him the first time, set him up with an experienced, patient queen who won't terrorize the poor fellow. Best if it's one he knows, too... and *always* supervise. Some cats just do not like each other.