Multitool are generally the way to go.
Pair it with a light fixed blade, hawk, breaching tools or whatever feels appropriate for field work.
Examples
Leatherman juice, victorinox swisstool, leatherman wave, victorinox soldier, leatherman MUT
Spyderco aqua salt, spyderco street bowie, mora 2000, kabar
If all he wants is a sturdy utility knife he can save some weight by going for a folder over a fixed blade... on the other hand the knife in a multitool will do most utility tasks just as well while also being multipurpose.
Folders
spyderco endura (3.6oz),
spyderco military (4.5 oz)
CS voyager large tanto (4.6 oz)
CS Recon 1 (5.3 oz)
Fixed blades
Spyderco aqua salt (4.6oz just knife or 7.0oz total with the sheath),
Spyderco street bowie (3.8 oz just knife + ? oz sheath)
Fighting knives are a bit of a sticky topic in that if the military thought you needed a fighting knife or something to get you out of a clinch they would issue it and train you accordingly. Some units are issued such knives and trained to use them... but the majority aren't. Because knife-fighting isn't a very practical thing for the rank and file. They have guns and friends with guns.
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