Can a man become unsterile that is sterile already?
It depends on why the man is sterile. There are many causes.
If he has had a vasectomy, an operation to make him sterile,
that operation can now be reversed for a majority of men, if they
want to go through the hassle and expense.
If a man is sterile for some other reason, there are many
possible medical reasons and many possible treatments. Only a
fertility expert knows all about it.