it should be "his being". That's how we use pronouns before a gerund. Also this question may be helpful as well http://www.gmatclub.com/phpbb/viewtopic.php?t=27429&highlight=helping
Correct, possessive pronouns are used before gerunds. But my problem is, how the heck do people recognize that being
acts as a gerund (noun) here?
I am guessing that the preposition tells you that it is a gerund, as examplified in this case as well:
Everyone of us has understood that without
his helping us we would not have succeeded in our program over the past six months.
Gerund means "the -ing form of the verb" and it is a noun from now on, not a verb anymore.
In this sentence, you should pay attention to the meaning. "his being" means "his existance". "him being" is totally something different..