My dad just found out he has prostate cancer and they say it is very small right now (the size of a pencil point). ......How fast can it grow?

Slowly. But much more important than the size is the grade, or gleason sum, assigned to the cancer by the pathologist evaluating the biopsy. Grade, psa level, your father's age, and other factors will help to define if this is a low, intermediate, or high risk cancer. Speak to a urologist & get multiple opinions. Many factors should be considered when deciding either treatment or active surveillance.
Usuially slowly. Prostate cancer is typically very slow growing. Whether the cancer will have any impact on his livelihood depends on how aggressive the cancer is (usually given a "gleason score" between 6-10, higher being more aggressive), and how old and healthy he is. Most men die with prostate cancer, not because of it. Nowadays we are more careful about trying not to overtreat patients unnecessarily.