This guy tried to steal my girlfriend and he needs to pay.
Most Helpful Guy
The CIA has a good handbook for that. The first mistake you made was to call it torture - that's illegal, even in the US. Pros call it "enhanced interrogation", just to be safe on the legal side.
Well then, on to my favorite methods of enhanced interrogation. I see that you don't want to extract any information, you just want him to suffer. That makes things a lot easier. I wouldn't recommend sleep deprivation, starvation, dehydration or any other methods that are primarily focused on breaking the subjects will. You can skip to the painful part.
Waterboarding is a classic. If you do it right, he'll think he's going to drown, but he can't. Takes at least 7 or 8 stages of that before most subjects get used to it and stop panicking.
Another favorite of mine is to push a sharp object under the subjects fingernails. Relatively easy to perform and extremely painful. Be sure that he's properly tied up though, otherwise he may be able to take the sharp object from you and use it as a weapon.
I have some more if you're interested, but we should probably do this via private message. The people here are easily offended.1