wait til he is about 2. that way you can have the vet xray his hips and make sure he doesn't have problems. it will also give you a better idea how he is going to look but i'm told they don't reach full growth til about 3 years
Definatly wait until at least 2yrs. Hips and eyes should be checked on both dogs before breeding. By checked I mean certified by (hips) OFA, and (eyes)Cerf. Most of the large breeds are prone to hip displysia, and should be checked.
you also might want to check and see if there is an americal bulldog club. They may have some info on other possible genetic programs that you should watch out for.
Alot of states are now enforcing a puppy lemon law that makes the breeders responsible if a puppy has any genetic problems. So in the end checking for health is a good thing. Good luck, and when all is said and done, enjoy the smell of puppy breath lol