Hi Rachel
Welcome to the group. Although your boy may be well behaved and have good markings, this would not in itself make him a suitable stud dog. Although markings are one consideration it is more about overall conformation, bone structure, muscle tone, overall size, dentition etc etc etc. being well behaved is good for you - but it comes down to what his overall temperament would be, Also having a good pedigree is no guarantee that he will be good for stud. Even the top dogs/bitches can produce progeny that is well below standard it all depends on their genetic make up.
At 6 months he is too young to decide if he is any good. Your vet will try to pursuade you to have him castrated, it's what they do generally.
There are a number of questions you need to ask yourself before deciding if he should be used for stud and a good person to talk to would be his breeder.
Have a look at this site
click here for lots of information about just what breeding entails. It also includes the responsibililties of the stud dog owner. It's much more than just letting your dog have a bit of fun!!;)
From what you have said though - if he is to be a family pet then he's not going to prove himself in the ring or field then reputable breeders are unlikley to be interested in him. If I were you, I wouldn't get him castrated just because the vet thinks you should - look into all the pro's and con's of it but put thoughts of using him for stud away and enjoy him as a pet.
hth
Pam n the boys