This is the first I've heard about PCa causing ED. I was diagnosed just over 2 years ago at 68, with a slightly more advanced cancer, a bigger prostate and PSA of 9, but had absolutely no problems getting an erection at the time.
I couldn't have RT due to suffering from colitis, and surgery was only partially nerve-sparing, so I do have ED now. However, I have an undetectable PSA and practically no incontinence, so overall I'm pretty happy with how things have turned out.
I would ask for an honest opinion about whether hubby is ever going to get full function back whichever option he takes. After all, if he is going to suffer from ED anyway, it isn't worth worrying about that particular side effect.
However, I agree with Lynn; there are other options to surgery/RT these days and I would see if one of those is possible.