Get yourself either a drill bit adapter or a purposefully made string winder, makes it so much easier especially on pita instruments like Mandos and Banjos. And not the manual powered ones, you want something that spins them quick. If you get the drill bit one make sure to set your clutch really low and stop using it when you're getting close.
I'm a guitar player and I will say, it is hard getting musicians gifts because so much depends on their personal playing and fashion styles. Plus you don't know what they might already have.
For example, yeah getting him a notebook is thoughtful, but what if he doesn't like the way it looks? Or just isn't a notebook guy? I know I personally wouldn't ever use a notebook, and I'm way older than him.
You could get him strings but there are so many different sizes and materials that you might get him a type of string he can't use. I do think this is a great gift IF you are positive you're getting the correct type, but you'd have to ask him and spoil it being a surprise.
That said, I have gotten some music related gifts that I really appreciated. One year, I got a really nice guitar strap. Like a super heavy duty, thick, durable leather one. I love the way it looks and feels, but there's a bit of a risk that he might think it's ugly. Again, fashion as a gift is always risky. But this is a very practical gift that can make his life as a guitarist much better!
Another was a string winder bit. Basically, if you have a power drill, it's just a little chunk of metal and plastic you stick into the chuck (instead of a screwdriver bit) that can wind and unwind your strings quickly. Changing strings manually takes forever, the bit reduces it down to 2-3 minutes. But they're only like $5, and he does need to already have a power drill (another very practical gift for anyone entering adult life).
Other practical ideas are a big bag of of picks (thin or medium gauge), a stick-on pick holder, nice instrument cables (I like monster brand, 12-foot), a clip-on tuner, or a nice gig bag.
this Guitar winder drill bit is cool as hell. When you change strings, you just use an electric drill for 2-3 seconds instead of winding and unwinding for a long time
A real time saver when you’ve got a bunch to do at once. Link