Ompare the following two cases:
(1) If he was from another planet ...
(2) If he were from another planet ...
Clause (1) says "He may or may not be from another planet, but let's consider the case in which he, in fact, is". Clause (2), on the other hand, says "He most certainly is NOT from another planet. Ha ha,
That's impossible. Still, let's just imagine what would happen if that impossible circumstance were realised".
In other words, you say WERE to imply that the condition is impossible. As in "If I were you..
If he were from another planet; if he were God;