You can produce, and you can reproduce.
yes you can (I did)
No
Yes
If he has a testicle, there will be sperm.
Yes.
Yes
Birth defect? Genetic mutation?
Yes, as long as the testicle is functioning properly then you can reproduce with one testicle.
The overall Count will be a bit lower, but if that testicle is healthy, the man will still be fertile.
monorchid
orchiopexy