A couple of things:
1. Start doing Kegel, "pelvic floor" exercises "before" the surgery. It will give your husband a little head start.
2. Diapers or pads? It is dependent on what type of underwear he normally wears. "Pads" to do not work in boxer shorts...they will fall out, especially when they begin getting heavier with collected urine. Pads work best with jockey undershorts. Diapers work best with boxers.
3. Don't make the mistake that I did: When I was back out in the world after my catheter was removed at the 11th day post-surgery, I was leaking pretty badly, so I thought that I'd be "smart" (nope) and put a Depends "pad" inside my Depends "diaper" and slip my boxer shorts over that, plus my casual attire shorts. I needed an easy way to slip into a bathroom, pull out the soaked wet pad, and replace it with a fresh one that I carried in my pocket. Here was the accidental epiphany moment: I was out on errands quite a while. I used my back up pad, and it too got soaked during the next hour that I was out on errands. I couldn't stand the wet, uncomfortable feeling of that soaked pad, so I pulled it out in the restroom, and wore just the diaper until I got home an hour later. I immediately noticed that I stopped leaking without the pad in my diaper! The "epiphany" moment was that I realized that the thick pad really was "too much" down there...it occupied too much space within the diaper, and it pressed up on my perineum and therefore pressed against my bladder, causing the very (excessive) leakage that I was trying to avoid. From that day on, I stopped wearing a pad inside my diaper. Life has been great. I can go many hours (2-3 diapers a day, so one diaper slightly wet every 8-12 hours) with very little leakage now. Part of this success is also due to the next factor:
4. Make sure you husband's urologist writes an order for Pelvic Floor Physical Therapy (PFPT). It is a subspecialty for physical therapy, mostly for postpartum women to regain their continence, but many post-RP men go as well to get that extra help with Kegel's and many other exercises that strengthen the pelvic floor. There are some pretty wild exercises beyond Kegel's that help.
I read in Dr. Patrick Walsh's book: "Surviving Prostate Cancer" (the "bible" for men with this issue), that we have three sphincters between the bladder and the tip of the penis (there is not one at the tip though). The nature of RP surgery, is that two of the three sphincters that control retention and control of urine and urination, are removed with the prostate. The "one" sphincter that remains, is the weakest of the three, so it really needs to be re-trained to help retain urine and stop leakage. I read a statistic that roughly 2/3 (66%) of men regain most/all of their continence within the first 90 days post-RP. I can say that I am not perfect yet, but I am 95% there at four months post-RP. I still have my accidents..what is called "stress incontinence"...doing every day little things like standing up, or sitting down (especially in my car), and sometimes spontaneously while walking. I just make sure that I urinate before I leave the house, and that I am really empty before leaving. I usually urinate, then do a couple more quick things for five minutes, then urinate one more time. There is always another 5 - 15 ml that somehow comes out, even though I thought I had emptied my bladder minutes before. My PFPT even says that when home, I should "sit down" to urinate. It is better/easier on the pelvic floor muscles and helps you really get every drop out. I usually forget to do that half of the time, but I am trying to do better. It is just a weird thing to do as a man...men stand, women sit. It is weird, but I try to sit. Part of PFPT also includes a close examination of when and how much liquid you drink. I am on a strict, modified liquid consumption schedule: no caffeine, no carbonated soda's/beer, no acidic drinks like my favorite OJ, and only 8 oz (1 cup) of "water" at any given time. I am to "time" consumption of that 8 oz with urination, and I am to urinate every two hours whether I feel like I need to or not. That all has really got me under better control. I break the rules and take little sips of my favorite decaf sodas just to get "flavor" in my mouth (drinking water 100% of the time is impossible for me, and likely everyone else). But all of that actually helps. Your post-surgical bladder gets angry...pissed off (pun intended)...with caffeine, carbonation, and acidity, and will cause you to leak. The bladder is irritated with those things, so water is the preferred drink each time you drink that next 8 oz. I just find drinking water all day, every day, nearly impossible and boring.
I hope this helps the overall pre-surgical situation, as well as the post-surgical expectations and tips/tricks to have a minimum of frustration with leakage. Good luck.