My two cents?
Be honest with him, like you want. That's my favorite way to go. Haha. But I would wait on it until he gets the rest of the sex talk. When he gets the birds, bees, pregnancy, condoms, STDs, HIV, blah blah, I'd throw that personal piece in there. Most CF men (I think it's 98%?) are infertile. The sperm is in there most times, it just can't get out due to lack of tubing, blah blah. Many CF men are having biological children through IVF procedures, blah blah.
Your husband does have a good point, though. So be sure to hit home that some CF men can get a woman pregnant naturally (there's at least one CF father on here who did it the natural way -- I forget names), so he had still better be careful.