In order for a son to become a jr does he have to have the same middle name as his father?

Answer 1

Yes. "Junior" signifies that the entire name is the same. The only difference is the use of "Jr."

If the whole name is repeated, but it's not the son (let's say it's the grandson), then it's II ("the second") and not "junior."

Answer 2

In informal direct address, many sons who do not have the exact same name as their father may be called "Junior," when children. However, the formal use of "Junior" as part of a name is only of value when there is another person with the same name, who would be called "Senior." Most commonly that person is the father, but in small communities it may be an uncle, the grandfather, or a great uncle.