Hi bbm, this is my very first post ever but our situations are so similar that I had to speak up. My husband is a survivor of a bone cancer that he had at age 16 and like your husband had full body irradiation as well as chemo that decreases fertility. I knew his fertility was in question when we started dating 10 yrs ago but he was so wonderful that I never thought twice.
Last year we started very early stage ttc planning, and he got a semenalysis which showed a zero sperm count. It was not unexpected, but I'd adopted the same optimistic mind frame as you're talking about and it was a big blow. I cried a bunch, it was all more than I expected given how well informed I'd been for years. So, I'd tell you don't give up hope and dont stop bing optimistic, but just know that even though you have all the info, you'll still be really emotionally affected if you get actual medical confirmation of absolute male infertility. I never found the stats either - they're really poorly studied - but our doctors told us anecdotally that they were less than 10%. But your experience may be different... The different drugs and treatment have significant differences.
But it's not all bad, fast forward one year and we're just about to start our first iui cycle with donor sperm. my husband was super involved in donor selection and is fully on board and psyched now. Our re requires a counseling session for any donor tissue procedures, it was super helpful and we haven't looked back since. My husband's concerns about not being the "real" father were really eased after that meeting and he's just excited and ready for a baby now!
Okay, sorry so long, just wanted to let you now you're not the only one. There are highs and lows but you will get pregnant one way or another!