He was your first boss or he is your first boss?

Answer 1

"He was your first boss" if you are writing/speaking in past tense, like telling a story.

"He is your first boss" if you are writing in future tense, like writing a guide/walkthrough.

If the person is still alive, you would not be incorrect in saying "He is my first boss." If you say "He was..." it might be presumed he is no longer among the living. It is not unusual to hear your boss' boss called your second boss. That would make your immediate boss your first boss.