Do you want him to conceal a regular knife or do you want some sort of trick knife?
You can find knives disguised as belt buckles, knives disguised as hairbrushes, knives designed to hide in boots, knives designed to be worn around the neck (in front, typically) knives disguised as hair picks, and all sorts of gimmicky ways to disguise the actual knife or help you hide it.
There are also credit card knives and lipstick knives and a bunch of other stuff.
For hiding spots, besides the belt buckle knife, you could hide a small knife in the belt itself. There's also the popular boot knife. A knife in a sheath under the armpit. A knife in a "crotch sheath," a knife taped to the back of the leg or the arm, etc.
Obviously, some are more accessible and effective than others.
As to throwing a knife, throwing knives are often oddly shaped and seem like they'd be harder to hide. Look at some pics and you'll notice they aren't typically simply straight. They have a bulge in the middle to help with balance. That makes 'em a little bigger and harder to hide.
Is the character expecting to get searched? If so, how well trained is the person doing the searching? Cops and ex-cops and trained security know the best hiding places for regular knives and are aware of many of the trick knives out there, like belt buckle knives or hairbrush knives, etc.
There is a way you can work that to the character's advantage. If, in the story, he had been previously searched by the nemesis security he could use that knowledge he gained to his advantage.
For instance, he could notice that they made him go through a metal dectecter or used a metal detector wand on his body and only did a brief pat down.
If he realizes that metal detectors are a big part of their security strategy he could use a plastic knife (not the plastic silverware kind) to sneak it past the metal detector. I don't think that would throw well though.
