Welcome to HW. Seems like you have arrived here the hard way.
I am a fellow Gleason 9 (4+5), and had 9 of 12 biopsy samples/cores come back positive. My PSA was 7.2.
The PSA is whatever it is. There is no right or wrong. You can have a very high PSA (even into the thousands) without PCa (prostate cancer), and a very low one, but have agressive PCa. It will help the doctors with statistics for planning treatment. If you had arrived before the biopsy, there would have been a suggestion to do a round of antibiotics just in case the PSA was generated by an infection. Just to understand here that the PSA is one of the indicators, not definitive on its own. But the biopsy is positive, so all that discussion is to the side now.
It is more "interesting" to know what his PSA has been over the last few years (has it been steadily or quickly increasing).
I had surgery, then radiation therapy (RT, IGRT, IMRT). Someone made a comment that it was late for surgery. That would only be a speculation, and if your FIL was over 70, perhaps true (my uro/surgeon suggests only RT for older men due to the side effects of surgery). There is a significant discussion your FIL has to have with his doctors to understand the side effects and values of the various treatments they will offer.
Aggressive means that the PCa is likely to spread, as opposed to Indolent, which means it may sit and cause no real issue for the rest of one's life.
With those who know about my PCa, I am very open, and little, if anything, is off limits. However, because of my job situation, there are many with whom I do not and can not discuss it. I know many men who just refuse to talk about it at all. You will just have to find your way by offering to be supportive, and open to talk if he wants to.