
Before you put him to stud he will have to be hip-scored and eye-tested, to make sure he won't be passing on any hereditary conditions for which you could be sued. Bullmastiffs have quite a high average hip-score (not good) so the chances of legal problems are higher than in many other breeds.
Stud dogs aren't usually advertised - they are seen doing well in the showring and bitch owners ask to use them. Bear in mind that being used at stud, especially only once, can turn a well-behaved family pet into a very different animal (leg-cocking indoors, agression to other males etc), especially when there's a bitch in season anywhere in the area!