Yes
You can produce, and you can reproduce.
Yes, as long as the testicle is functioning properly then you can reproduce with one testicle.
You would ejaculate less sperm cells. That is not too bad since you can still reproduce. In some cases, you might need some testosterone supplementation.
If he has a testicle, there will be sperm.
Yes.
The overall Count will be a bit lower, but if that testicle is healthy, the man will still be fertile.
monorchid
Testicular torsion can affect fertility, but the outcome depends on the promptness of treatment and whether the affected testicle was salvaged. If the torsion was treated quickly and the testicle remains healthy, there may be no significant impact on fertility. However, if the testicle was lost or severely damaged, it could reduce fertility, though many men with one healthy testicle can still father children. It's best for him to consult a healthcare professional for personalized advice.
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.
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.
If you mean that one of your testicles was removed (and you now only have one), yes, it is still possible to have kids. With one testicle, there is something like a 60% chance that you will still be able to have children. You need to get fertility testing from a doctor to be sure.
When a man has a testicle removed by a doctor, he may opt to have an implant put into the scrotum in order to make it seem like he still has both testicles. The implant does nothing other than occupy the empty space where the removed testicle once resided.