The best gifts are things we want, but would never buy for ourselves. Often frivolous and unnecessary, but they are things we would love to have on the spur of the moment, if we had too much money in our pockets.
That leaves a lot of room, I realize, but you know him better than we do. As stated already, without knowing what he already has in the way of tools, it is impossible to make a solid recommendation. I am quite sure my wife would not know where to even begin to look if she were to want to buy me any sort of tool. But then I have a business and have been doing this a long time. Tools I got, gizmos and goodies? Can always use more. Besides, it's the thought that really counts.