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Anything with serates isn't going to work.. Can't resharpen those personally. I've tried on a few knives, but never really the same afterwards. Need to stick to a regular blade, I know he likes straighter edged blades; but that really limits knife selection. He would never wear a neck knife. Hell, He doesn't even like wearing necklaces at all.
Thinking a Spyderco Manix or something of the sorts with a REALLY good edge retention steel?!
Overall, the Manix's S30V is a good balance of edge retention, sharpenability, toughness, and cost... As well as several other steels out there. If you decide to go with very high retention steel, like ZDP-189 or others, just try to decide if the added effort and stuff associated with sharpening would be an issue or not.
Maybe something in 3v? Just a thought, heard it holds an edge better than most and still not too bad to sharpen, just my opinion, correct me if I am wrong or pass add. What about an Esee 3 or 4 or Izula?
Speaking as a vegetable farmer, the most useful knife I ever carried was a drop-point Spyderco UK Penknife. Lightweight, slim, easy to operate, and the choil prevents it from closing on your hand during use. BD1 doesn't retain an amazing edge, but it's absurdly easy to touch up.
Otherwise, I'd say Native 5 lightweight in S35vn. Good edge retention, easy to resharpen, and supremely ergonomic.