Claire Perry,
I have 2 male GSPs that have previously been used for Stud. I would ask that you think long and hard as once the Dog has been used it changes the animals behaviour around Bitches and other Males.
I would also ask why this Dog!, is he an Exceptional worker with a proven breed line or do you just wish to have pick of a litter for yourself.
I would ask the vet to Hip Score him and check him over fully for any disorders and then if he is fit let nature take its course. I would ensure you choose only best quality Bitches for him as poor breeding is bad for all concerned and will not enhance the breed also the mating agreement must be clear to both parties before mating takes place.
Think long and hard.
Dalkur