How is he going to get from the cargo hold to the passenger compartment? I believe they are separate compartments without a door between them. I'm sure there are experts that know more here. I was under the impression that these areas were at different temperatures and pressures as well.
I would be more concerned with somebody shipping a bomb as freight. I have heard that little of the freight that is also in the cargo hold is inspected. I can vouch for this on at least one instance. I shipped some hardware on a Saturday morning this past March using airline freight for a customer that needed it ASAP. They gave me their airline freight account number and the shipment was accepted without question, weighed (about 60 lbs in 3 boxes) and placed into a shipping container - the kind shapped to go into a commercial airplane. I do not know if there was further inspection but it did not look like that would be the case.
Some are, and some aren't. There are cargo planes that have pressurized cargo areas, as is obvious by the fact that the stow away in the story didn't end up as a bloated popsicle. But there are certain configurations that would carry the freight in a manner that would make it possible to get to the cockpit area ... he may need a crow bar to open the door, but that is just a few more pounds of 'cargo' in the crate with him.