http://blausen.com/?Topic=9336
There are lots of other medical animations on the Blaussen site that might interest you as well.

what happens during male orgasm
https://www.youtube.com/watch?v=B3GRJGV9YNQ
what happens during male masturbation
https://www.youtube.com/watch?v=F5QK8MZeiA8

It is my understanding that if you don't get it up you will run the risk of not having fresh blood and oxygen getting to the muscles and they will suffer and go into decline.
You should probably use some of it just to keep things going until you can use it with your wife.

You would probably find suggestions in the book "Saving Your Sex Life After Prostate Cancer" by Dr. Mulhall.
Whatever you do you need to try to keep "new blood" coming into the penis with the hope that it will "wake up" after a while.
Courage, man.

There is a radiation treatment in Canada done by the Cleveland Clinic (not yet approved by the FDA, but done in many other countries). Of course because it is in a private clinic it is on your dime. http://my.clevelandclinic.org/canada/services/prostate-cancer There are several good books out for gu...

Also make sure you get your tests done by the same lab and at approximately the same time every time. Either one of these can cause a fluctuation. Are you also aware that diet and exercise has an effect on keeping the cancer at bay? The Prostate Cancer Foundation has a good booklet on this. I recomm...

Are you taking Flomax (tamsulosin) to keep things open down there?
If you are prone to kidney stones, it is good to drink water during the night and go several times during the night. Helps keep the kidneys flushing.

Night and especially early morning is when your testosterone is highest. That might have something to do with your erections during night, but not
at other times. I had radiation and prostate seeding 4 years ago and I can get good wood many early mornings, but I don't last long at all.