Interrogations and torture sessions work because usually there is one guy getting the torture, and multiple shifts administering it, not because of it's lethality.
Shift A will do the work for 5 hours, then spend the remaining hours of its work day filling out paper work and relaxing before going home to the family. Shift B picks up the next 5 hours. Shift C does so as well, and Shift D does the next 5 hours, and Shift D does 4 hours. All of these shifts relay to each other what is working and what has failed, and what the interrogated/tortured has said / how much he's been tortured.
Researching the CIA guide to torture/questioning is probably a good bet (although keeping the practices in your time frame would mean not using all of their methods). One of the key points of the book is that a subject is far more likely to break simply from seeing the torture implements for extended periods. The moment the subject actually gets hurt, he achieves an awareness of the pain caused by it and can adapt if he is stubborn to it. This adaptation is a victory for the subject, and that gives him hope or fuel to bear the torture. Torture and interrogation is very much about denying all hope and all chance of winning to the subject.
Personally, based on all that, I'd have shifts using psychological torture on the PC. This can mean Profession (torturer) checks opposed by Will Saves. If your guys are simply torturing him for the whimsy of whomever has captured him (ala "I want to hear his screams all day, for the rest of the month. Muahaha I'm so EEEEVIL. MUAHAHAHA") then just break out the BOVD and have their torture checks reduce the Con damage with high results. Retries are a definite.. over time the subject recieves penalties while the torturers, in shifts, do not.
Also, by doing it slowly you actually give the other PCs a chance to do try a rescue. Based upon how long it takes them to get him out, the character can suffer from all sorts of phobias etc, or gain a new hatred for the gestapo. Call of Chththehthnuml has all sorts of rules for making Sanity checks. Definitely think anyone tortured for long deserves to make a few.