As far as training him for riding I would do it like any other horse. The only thing that I would do different is make sure he knows that it is unacceptable for him to nicker, look, at other horses/mares. When he is under saddle he needs to have his full attention on you and you alone. If he is taught well by the end you should be able to stand in a crowd of horses and no one would know he is a stallion.

It would definately be better to teach him something new when his hormones are not raging. So doing it in the off season would be a good idea.