Hi,
You state that your dad has "plenty" of knives. So my guess is that unless he has "plenty" of cheapies, he probably knows a bit about knives. The fact that he knows about William Henry and specifically mentioned that he would like one probably means that he his familiar with their designs, materials, and the artistry (and they are fully functional works of art) applied to making them. The danger for you, admittedly not knowing a lot about knives, is that you may think you are getting him a less expensive equivalent when in fact you are getting him a less expensive, lesser knife. I can't tell you how many times I've heard people talking about how they saved a lot of money and got something just as good when it was simply not true. One can force a butter knife to cut raw carrots - it does serve the same function as a nicer kitchen knife, one just won't have the same pleasure or see the same quality of the cut. Maybe not the best analogy.
I'm sure your dad would be happy with any quality knife you give him and there are a lot of great knives out there (and suggestions in this thread), but (and I know this sounds like a sales pitch - it's not meant to be) I think you would really see him light up if you actually handed him a nice William Henry - finances permitting (I know were talking about a considerable amount of money here and I don't take that lightly either). William Henry knives run from about $250 on up to the sky and they (William Henry) do a pretty good job of keeping the prices the same from dealer to dealer unless it's a dealer exclusive. From what you've stated, it sounds like he would like a somewhat fancy one so you may want to look at more than the $250 to $300 EDC models if you can afford it. My suggestion would be to really study the William Henry's as far as design, materials, artistry, etc. As you've seen, Knife Art has quite a few and you can learn a lot by just reading the description for each knife. You can also go to the William Henry website to continue your education. I believe they have some nice videos showing their processes. Once you know what goes into the William Henry's, you can use what you've learned to compare other knives and maybe even contact some custom makers to discuss the possibilities.
There are quite a few quality production knives out there as well as very skilled custom makers here on Blade Forums. Be aware that if you're asking a custom maker to build a William Henry-like knife, it may take some time. If it were me, I would either take the short cut and get your dad a William Henry if you can swing it and be assured he will be delighted with it or let him work with a custom maker to turn his ideas into something solid - maybe give him the choice if appropriate. I apologize for seemly pushing the William Henry's, I just feel that they fill a rather unique niche in the knife world and if that is what your dad is hoping for, he might be most delighted receive one. As far as clips go, the EDC models have them but many of the more expensive WH's come with a nice snug leather pocket pouch/sheath that have the clip. It's kind of nice not to clutter a really nice knife with a clip. $600 to $800 will get you a pretty nice one. Hope this helps and have fun with it.
Mike
P.S. I came by my two medium range WH's because my wife has no idea how to shop for me and usually asks me to shop for my own birthday and Christmas presents - obviously, I take full advantage of the situation.