Considering the knife he lost was a larger one, 3.7" (or 3.15" for the mini version) I would definitely recommend something in similar size.
Also, consider if in fact he did like/prefer the partially serrated blade? If so, a partially serrated blade would be something you'd want to get.
Also did he prefer the spring assisted opening?
The H&K axis is roughly the same size as the Trident, available with a partially serrated blade, but much better quality and d2 steel blade.
The Kershaw link is 3.25", closer to the mini-Trident, the houseofblades s35v steel exclusive model (red or blue handle) is a great deal, great steel, assisted, but is not available in partially serrated. The standard 420hc steel model however is. There is also a Damascus version.
Another great option, as others have mentioned the 3.375" kershaw blur, (assisted and available in partially serrated), is the new Kershaw dash, slightly larger at 3.5", without the recurved blade, not available in partially serrated, but a much better edge holding steel (n690) then the standard Blur, (the s30v version of the blur, another eeven better edge holding steel, doesn't have a serrated option), and the dash has a flipper tab, both are available in different colors including plain black.
The Kershaw shallot also is a good assisted choice available with a Damascus blade or a partially serrated blade, (not both). 3.5"
I rescind my earlier suggestion of the Buck marksman, not that it's a bad knife, but realizing after rereading that he carries in his back pocket, the lock system on that one will likely shred his back pocket in time sitting on it. One more I want to throw into the mix though is the Buck Vantage Force Pro.
Only 3.25" in blade length, not quite as large as the Trident but a bit larger then the mini version, non assisted but comes with a flipper tab, s30v steel, and a partially serrated option...
Based on the Trident he had, my top 2 suggestions are the H&K Axis (made by benchmade) or for something a tad bit smaller, the Buck Vantage Force Pro,
both with partially serrated edges, but also much better steels in the blade to hold their edge longer over the aus8 used in the SOG.
A lot of great options in this thread to choose from, be sure to come back and tell us what you got for him and how he liked it.
(FYI, Some companies, like knifeworks, also offer engraving services if that's something you'd want to consider as well to personalize the gift. Just saying).
Best of luck!