If you ever need to figure out whether you should use who or whom, replace the word with he or him (or whatever subject/object pronoun pair you like, except for you and it because they're the same). If it sounds right with he, use who because it's a subject pronoun. If him sounds right, use whom because it's an object. You might have to rearrange the sentence a bit to figure it out.
Examples:
- ____ cheated in the last election? Him doesn't sound right, he does. Subject pronoun, use who.
- ____ did all those stacks of illegitimate ballots vote for? Rearrange to "All those stacks of illegitimate ballots did vote for ____." He doesn't sound right, him does. Object pronoun, use whom.
In your case, "by he" isn't right, but "by him" is. Prepositional phrase, object pronoun, use whom.