Yes, you can produce semen with one testicle.
You can produce, and you can reproduce.
yes you can (I did)
No
Yes
Yes
Yes.
If he has a testicle, there will be sperm.
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