Need to know what kind of fish he's cleaning. It's important to get the correct blade length. The better filet knives come in short, medium, and long blade lengths (approx. 4, 6, or 9 inches). I have and have used most brands of fillet knives. Dexter Russel with the Santoprene grips are great, used a lot commercially, but you don't see them in knife stores this BladeForum group would frequent. The Bucks are good but when they dropped the vintage USA models the new styles have an inverse curve near the handle that makes sharpening too difficult, and also the thin blades of their present models are too flexible.
Today the line of Kershaw filet knives is one of my favorites, worth taking a look at. Rapala is ubiquitous but I think they are really overrated, not to mention difficult to sharpen (my EdgePro does a great job, but not everyone has this superb device).
You want a handle with good (larger is better) grip that doesn't get slippery when wet / slimy. The Kershaws are also good here.
Finally, In my opinion it's much better to have several sharp knives on hand ready to go than to fiddle with 'tuning' an edge during a 2-cooler fish cleaning exercise when the fishing was good. Thus, sharpen your knives at home on good stones, don't go in for 'kits' that include a $2 quasi-sharpening device. Fish skin / scales / bones (as in 'Tuna') will dull a good knife after an hour of cutting. The meat itself doesn't dull a good knife.
