Can his sperm be taken?

Figured that would get your attention. Ok, don’t know where this come from sometimes.
My nephew has a disorder that from birth, has left him with no muscle control. DId not think he would not live six months but is now 21.
The jury is out (figure of speech) if he is a Steven Hawking or of little intelligence. He does seem to have a sense of competence from the
way he reacts to his enviorment and the people around him.
NOW: This is a hypothetical question. If his father wanted to have a legacy. (the passing on of the family name)
But in order to do this his father would need to extract sperm from his son. Even if the mother was also ok with the idea. Could it be done?
It is legal and/or ethical?