The simple answer seems to be that we use 'he were' in formal settings and 'he was' in informal expression. However, the use of 'were' seems to be the general choice if in doubt.
Why?
It has to do with the use of the subjunctive mood being used after words like 'if', 'as if', 'wish', and 'suppose' and we use the past subjunctive of the verb 'to be' instead of the the past simple.
You can get a really in-depth explanation from the Grammar Girl if you really want to understand the idea behind verb moods.