Depends on the causes of the hair loss, but if indeed it is androgenic alopecia then
I believe Proscar is Finasteride at 5 mg, The smallest dose that has been shown to be effective in the Medical journal is 1 mg ( which is standard for Propecia) Every study I have read says that the effective dosage varies.
I am not a Doc but I would hit it at 5mg a day this should help reduce any prostate growth AND take care of the hair loss.

Terms you could google are Prostate Hyperplasia and Finasteride