"It is probably him who has it."
Translation:Det er nok ham der har den.
Better English is "It is probably he who has it": this sort of formulation is very formal in English, and with such formal phrasing "book grammar" is necessary. Yes, we all say "it's me" and so on; but if you're going to go all formal and say something like "It is I who am looking for you", then you've got to get the book grammar right. Normal spoken English, of course, would be "I'm the one who's looking for you". Likewise, no-one would say "It is probably he who has it" -- one would say, "He's probably the one that has it/that's got it".