well you have to look at the differences between whom and who.
If you can restate the sentence in your head and say HIM ..that sentence can hold a WHOM.. Because both terms (him and whom) are possesive.

then you have HE and that is paired with WHO. Again, if you can substitute he into the sentence when referencing t the person you can use who. non possessive ..

so you now have he who in you sentence.. and you wouldn't be able to say he whom..