i would think at 8yrs old if you do geld him, its not going to make much of a difference. his studdy behavior is already embedded in there by that age.
BUT BUT BUT- does he even act studdy? ive had many many studs that you would not even know they had their junk if you didnt look. some horses just dont act studdy. so this also has to be thought of too. studs arent as bad to control as a lot of people may think they are. as long as theyre kept in check and are paying attention to YOU and not whats between their legs, theyre typically fine.