My dad is bald. It used to be by choice but now, its um... doesn't grow back?
The term of what he is going through is called MPHL male patter hair loss
Uh oh that sounds like its not ever coming back!
It may or may not come back.  In males the hair loss often presents a receding hairline 
I see. Are there treatments for it?
I have found one treatment its called minoxidil, or a hair transplant surgery are the most benefiting treatments 
Hair transplant surgery? I had no idea that was a thing!
the condition. Otherwise, treatments may include minoxidil, finasteride, or hair transplant surgery. Evidence for finasteride in women is poor and it may result in birth defects if taken during pregnancy.
Pattern hair loss by the age of 50 affects about half of males and a quarter of females. It is the most common cause of hair loss. 
Signs and symptoms.
Classic male-pattern hair loss begins above the temples and vertex (calvaria) of the scalp. As it progresses, a rim of hair at
Frictional alopecia
Frictional alopecia is the loss of hair that is caused by rubbing of the hair, follicles, or skin around the follicle. The most notorious example of this is the loss of ankle hair among people who wear socks constantly for years. The hair may not grow back even years after the source of friction has ended.
Cause.
Hair loss on men's legs went largely undiagnosed, but is now thought to be quite common. While the overall causes are still being explored, the primary culprit is