I would say get him two blades, a smaller, fully serrated sheep's foot or hawkbill from Spyderco.
These come in super handy for cutting synthetic material like seatbelts and the crap crack heads store their junk in.
Next I'd get a Microtech Ultratech or SOCOM Elite. Those wouldn't be bad for self-defense and they're great for general utility.
They would also be a lot of fun to play with if he's gotta sit in the middle of no place for hours waiting on someone to break the speed limit.
I realize these aren't cheap so my final suggestion would be a serrated TDI from Kabar.
This would cover everything a LEO would need and it'd be cheap.
Lots of LEOs I know prefer knives and guns that are somewhat "throwaway" equipment where it's good enough quality to do the job but no big deal if lost, broken, or
confiscated for evidence or court or whatever.
If you get him any quality knife from a quality company it will be leaps and bounds better than most of the military surplus/ gas station crap I see a lot of military and LEO guys and gals carrying.
Spyderco, Microtech, Kabar, Cold Steel, Benchmade, ZT etc. take your pick, you can't go wrong.
Hell, get him a knife every year for Birthday or Christmas, he'll look forward to a new knife every year and then he'll have plenty of options for what to grab when it's time for his shift.
Also, we wish him the best and God bless him every single shift, it's freakin dangerous out there and LEOs are NOT, in my humble enough opinion, paid enough for what they do. I was an EMT/ Firefighter for a little while and decided I'd be better off fixing air conditioners and wiring buildings. For me, it just wasn't worth what you put up with for what you get paid, but God bless those that step up and protect random strangers regardless of the money.