Einstein kindly tell me whether him or he is appropriate.Strictly speaking, "whom" would be correct as it's the object form. However, it sounds quite pedantic or old-fashioned and "who" is generally used nowadays for both subject and object.
"Whom" is used a bit more as a relative pronoun after a preposition in formal texts: "The person to whom I gave it...". The more spoken form is "The person I gave it to".
Thank you johngiovanniIt is interesting that when you do a search on "If it was not him, whom" you quickly come to references to grammar books written in the early nineteenth century.
Anyway, I will go for: "him", as "I would trust him" or "Would I trust him?" would be correct, not "I would trust he" or "Would I trust he?"
As Einstein said, it sounds old-fashioned.
If you do a search on ""If I can't trust him, who can I trust?", you will find examples of how contemporary speakers / writers would probably say it.