Whom or Who?

I'm having trouble trying to figure out which one to use in this sentence:

"he believes everyone is a 'phony' and that he is the only one who has remained a geniune human being in today's society."

Any help would be appreciated, thanks.

As a rule of thumb, use who when "he/she" would make more sense (upon rearranging) and whom when "him/her" would make more sense.

I think it's 'who'

Considering I am an English Language A2 and normally very good at grammar, this is the one rule that confused me

That is the same with me, I can never work out which one to use and nobody every corrects me so I do not know if I'm using the wrong one?!

I think it's because no one really cares about the rule and it's becoming obsolete. Whereas someone say "hey, your really nice" and all hell breaks loose

Haha, their are two many grammer nazis around these days anyway but all they seem to care about are the difference between 'your' and 'your'.

....