I'm sorry your son is facing this situation at his age. The good news for you is that he was diagnosed early and the initial Gleason score and overall pathology indicates a relatively low risk cancer at this point. I would suggest that you seek a second opinion on the biopsy results to be absolutely sure you are dealing with the stage of prostate cancer that you think you are. Your son's doctors can show him how to get a second opinion on the biopsy slides.
I had a similar initial pathology although I was much older (59) when diagnosed last year. I agree with some of the other comments that AS for a man as young as your son might be worth seeking more than one opinion on. I too considered it but was told that eventually I would most likely have to deal with it and there will never be more options for treatment than you have now at this early stage.
I understand some of the other comments about surgery being a common or even preferred treatment for younger men diagnosed with prostate cancer although I don't fully agree with that logic. It seems to me that regardless of the age you should go for the best treatment for your individual cancer while minimizing potential side effects that all treatments have. While younger men generally handle the effects of surgery more easily than older men and their lasting side effects are often less adverse, there are still serious and significant side effects associated with surgery that a man in his early 40s should consider very carefully before having their prostate removed. If you are considering surgery, please make sure your son's doctors go into great detail about possible side effects.
I elected to have radiation treatment known as SBRT (Sterotactic Body Radiation Treatment) delivered by the CyberKnife system. Like other radiation treatments, the entire prostate is treated. CyberKnife delivers the radiation in five treatment sessions, each about 45 minutes in length, instead of the 8-week protocols that IGRT and the Rapid Arc deliver. All of them deliver fractional radiation doses and all of them radiate the entire prostate. It is not true that radiation rules out future surgical options if they become necessary but it is frequently more difficult depending upon the type of radiation used.
SBRT, IGRT, IMRT all deliver radiation with a very high degree of accuracy which minimizes potential damage to surrounding tissue and organs. The primary difference between these methods is how the linear accelerator used to develop the radiation is mounted, how the individual systems compensate for movement of the prostate during treatment, and the amount of radiation delivered in each fraction. Side effects vary and if the do occur generally pertain to urinary urgency which tends to go away within a few weeks of treatment and can be treated with over-the-counter drugs like Advil or prescription drugs like Flomax. A small percentage of men have some rectal toxicity (bleeding in the stool) which also passes shortly after treatment. Some men experience some degree of erectile dysfunction months to years following treatment but this condition is responsive to drugs like Viagra.
As one responder mentioned, proton therapy is also a treatment option and as its name implies uses protons to bombard the prostate (again, the entire prostate is treated) and the physics of this treatment are complicated but it has been around for more than 20 years and has as good a long term track record as some other types of radiation or surgery but it is much more expensive.
In my own case, I had my treatment last July and have had no side effects at all.
It's very important that your son understand the differences between all of the types of treatment that are suitable for him now, the potential side effects, and how these side effects may effect every day for the rest of his life. Second or third opinions are always a good idea.
The long term prognosis for someone like your son who is diagnosed early with a relatively small amount of cancer is excellent and he can look forward to a long and productive life. If I were in his shoes, I would investigate treatments that minimized potential side effects.
As others have suggested, educating yourself with a good prostate cancer library and reading of the experiences of others is very important in sorting all these choices out. Your son's pathology suggests that he need not be rushed into making a hasty decision about treatment. I hope you both can take the time to fully investigate all the potential treatment options available and consult with the necessary experts to help you reach an educated decision.
I would also encourage your son to personally get involved on this or other prostate cancer forums. Not only will he learn more from first hand interaction, he will have the additional benefit of interacting with a large group of sympathetic men who have traveled the same path he is just starting.
Best of luck to you and your son.