As far as feeling young, there are many in your boat, so don't feel alone. I was 40 when I found out, 41 when I had surgery (and still 41)...
... so I am still in the phase of recovering my erection capabilities. My first orgasm without ejaculate was with a completely flaccid penis. It kind of hurt, and felt like a rubber band inside snapped. Had the same thing, a bit more subtle, the second time. Nothing like that now.
It is hard to say what the orgasm feels like, as I am using a pump with rings right now, and I think that blurs the sensation. But I can relate to what Pete says above, it seems more like waves than an explosion, and is more subtle and lasts a little longer. As far as feeling, I still feel like my penis is trying to ejaculate, just nothing comes out, so that part of the sensation is the same.
How do you know if you are done? When you or your partner are ready to stop, I guess. The body tells you, though. But same as before surgery, I imagine on occasion he felt able to continue after ejaculation/orgasm... That is possible post-surgery too (actually, with the pump, sometimes I am harder after orgasm).
Sadness: the recovery process, to be honest, has a lot of sadness in it, and certainly some may be part of learning how to have and give sexual pleasure post-treatment. But it gets better with time, and it sounds like with his current situation, he will have an excellent chance for long-term recovery of his erectile capabilities.
Relief? Sudden relief: as I said above, I think it's more subtle, maybe longer lasting. Having said that, it is often said that men focus too much on the ejaculation and not enough on the orgasm, which is separate, really.
Hope this helps!
Prostate cancer diagnosed: May 15, 2006 (age 40)
Gleason score: 3+3=6
daVinci radical prostatectomy: July 25, 2006
size of tumor: approx 1.1 inches
post-surgery Gleason score: 3+4=7; negative margins from surgery
number of pads/day at 3 months after surgery: 3 to 5
number of pads/day at 4 months after surgery: 1 to 2
first post-surgery PSA: 0
ongoing post-surgery treatment:
Cialis every other day, Viagra "on-demand", ErecAid pump daily Cialis every other day, ErecAid pump twice daily (when I can manage it)