He will probably never be as calm as a gelding while he remains a stallion, although he can be made to behave properly with manners provided he has consistent training and schooling on what is acceptable behavior and what is not. You have to manage stallions differently than geldings, and they tend to be more aggressive. That is not to say that they cannot be very usable horses.
This fact sheet will provide you with much more in depth information.
F3922. An Overview of Stallion Breeding Management
