Looking back, I see his PSA is being closely monitored.
Finasteride (or Dutasteride) will halve his PSA, but doesn't reduce prostate cancer risk. So while on these medications (known as 5𝞪-reductase inhibitors, or 5ARIs), he needs to double his PSA readings.
These medications shrink the prostate over a period of around 6 months. We often talk about the prostate using Testosterone, but actually, it prefers another androgen called Dihydrotestosterone (DHT), which is 5-20 times more powerful than Testosterone. It converts Testosterone to DHT at point of use using an enzyme called 5𝞪-reductase. The 5ARIs prevent 5𝞪-reductase from working, and thus prevent the generation of DHT. Then the prostate falls back to using the less powerful Testosterone, but that tends to cause the prostate to shrink.
DHT is also used by hair follicles. Loss of DHT can cause reversal of baldness (although not if you've been bald for years). It's used the opposite way by hair which grows at puberty, so may lose arm/leg/chest hair (doesn't usually impact pubes or beard). Loss of DHT can also impact libido and erections a bit.