You can produce, and you can reproduce.
If he has a testicle, there will be sperm.
Yes, as long as the testicle is functioning properly then you can reproduce with one testicle.
I think you mean undescended. One of them is still in the lower abdomen. This should be looked at and corrected. It is too warm for it to produce sperm and there is a real risk of it developing cancer.
You should still be able to have erections, sex and children. If you feel badly about this because in some way you feel inferior, don't be. Your single testicle can still produce millions of sperm. You can get a false testicle to replace the old one, if you haven't already. No one will be able to tell the difference.
Yes. One testicle will still produce millions of sperm.
Yes, a man can still have children with only one testicle. The remaining testicle can produce enough sperm for conception. As long as the testicle is healthy and functioning properly, it typically can maintain fertility. However, individual circumstances can vary, so consulting a healthcare professional is advisable for personalized guidance.
One testicle can still produce millions of sperm. Your sperm output is more likely to be affected by other things (tight or too warm clothing, diet, alcohol, drugs etc).
Absolutely - the man would simply be unable to sire children.
Yes
Yes.
The function of the testicles is to produce sperm and hormones. For more detailed info visit: http://en.wikipedia.org/wiki/Testicle#Function
Nope, each testicle both produces and stores the sperm.