Just like stated above Emerson's (many good choices) I like the Commander, ZT's, Spydercos, Benchmades etc. They are all good brands just depends what his taste is, if you know what that is.
What type of work does he do? Highway Patrol, City, Foot, Detective, perhaps a specialty unit that will make the difference. If he is an FTO or firearms instructor he may have different needs also. Maybe one of the new Leatherman Mut or VTac Multitasker Tools will be best.
Maybe a rescue knife with a seat belt cutter and if he is allowed to have something other than issued gear on his duty belt a folding sheath knife like a buck 110 ( pretty much any knife can go into a sheath) would be better.
Because of all the gear on my duty belt it is awkward to carry anything in my front pockets and they are pretty useless, sometimes when I do carry a knife in my front pocket it will bind on my gear and pinch my leg. I have to clip it on my pants appendix carry style. The only knife I can carry comfortably in my front pocket if my Case Sowbelly which takes some digging to get out.
Lots of choices and any high quality brand is a good start.