I don't recall any gifts either (then again, I didn't get gifts for graduations either), but I do think it is appropriate.
I do know that some troops/councils don't technically allow fixed blades, so I would check that before gifting him one. Then again, if he's already close to 18 and being "done" with scouting, that might not matter as much. But I do agree a good bushcraft blade would be great.
Another thought is what about a good hatchet/small forest/boys axe? Those are allowed in all areas of scouting as far as I'm aware, and will likely be held onto for a while. A wetterlings, hultafors, husquavarna, or GB might be appreciated for a long time to come.
My final idea is a good folding knife or multitool. A svord peasant with some custom scales could be memorable, as could many other folders.
And congrats to your nephew

. My oldest nephew contacted me this week for help with his second class requirements (axe/knife sharpening and use

), so its fun to help out with this.