Sorry to hear your news, Jillu. Gleason 9/10 is a very aggressive cancer, so treatment will be necessary. The fact that it's still localised to the prostate is excellent news, and means that treatment will be given with the aim of a cure.
There are two basic treatment paths: surgical removal of the prostate, and radiotherapy. They have roughly equal (and high) success rates. I'd suggest that you download or order the "Toolkit" from this site which contains a load of information about prostate cancer and treatment options. I'd also recommend phoning the PCUK nurses on the freephone number shown at the top of the page is you have any medical questions.
It's not particularly unusual for an MRI to be clear but cancer found in a biopsy - the same thing happened to me. The good news is that a PSA of 5 indicates that it's been caught at an early stage.
The main thing I want to say, though, is DON'T WORRY! Localised prostate cancer is not a death sentence. It's a curable disease.
Please encourage your FIL to join this site. It's a lot easier to talk to people directly rather than by proxy.