If he were you or If he was you

When discussing situations that are imaginary or hypothetical, formal English has the subjunctive mood. The subjunctive mood is a verb form that is used for unreal or hypothetical statements. It is made up of the phrases I were, he were, she were, it were, etc. You often use this form when you are being wishful. For example,

I wish I weren’t so shy.
I wish it were warmer outside.
If I were taller, I could dunk a basketball.
If John were a rich man, he could drive a fancy car.
He acts as if he were the one in charge.
John spends money as if he were a millionaire.

All of the above sentences use the verb were because they aren’t true; they do not describe reality.

In the first two sentences, I am talking about things I wish would happen.

In the third and fourth sentences, I am talking about situations that would happen if I were taller and if John were rich, speaking hypothetically.

And the fifth and sixth sentences are examples of unreal statements.

Many people ignore the subjunctive mood in conversation and writing and use the simple past 'was'. Whether this is 'incorrect' or just informal and casual is a matter of opinion.