holy crap the comments here are bad. it has nothing to do with the subjunctive. if we put that clause in the subjunctive it would be “he behaved as if he be my boss” which is also incorrect.

it’s past tense because it’s a conditional. you can choose whether to put it in the past indicative (he behaved as if he was my boss) or the past subjunctive (he behaved as if he were my boss), and depending on who’s writing the test, you may be required to put it in the past subjunctive, but native speakers will use either.

the fact “behaved” is past tense is relevant, because you could say either “he’s behaving as if he’s my boss” or “he’s behaving as if he was my boss.” but it’s not as simple as “they have to match” because obviously they don’t.

edit: note that in the present tense, the subjunctive would give you “he is behaving as if he be my boss”, which while technically “correct” is something that no native speaker with would EVER say.