my first thought is that "whom" is a object case of "who", and since the chap is the subject of this verb to be, then the object (or accusative) case would not be appropriate ... as for the use of which, that etc - these choices are not always clear to native speakers, there are some rules, but largely ignored! I wouldnt stress too much about this one!
New words, one handy idiom, and a 2-minute quiz — delivered to your inbox to keep your streak alive.