That's really awesome to want to get a gift to encourage your (I'm assuming younger) brother. Your instinct to avoid specific instrument gear or sheet music is probably correct since his teacher would probably want to be prescriptive about that stuff, but here's a range of alternative ideas:

1. Ways for your brother to track his progress like practice journals: https://www.amazon.com/Violin-Practice-Journal-Musicians-Journals/dp/1648240011,
2. Ways to record/share his playing- maybe some media storage (Brett gave Eddy something like this violin-shaped USB flash drive): https://www.amazon.com/Walnut-Violin-USB-Drive-College/dp/B092DTDM5V
3. or recording software or gear if you're willing to spend more and do some research: Maybe a mic if he doesn't have one? https://www.sweetwater.com/store/detail/P170--akg-p170-small-diaphragm-condenser-microphone (as a bonus future gift idea, if he did some recordings, there are places where you can get custom vinyl pressed, if you think he'd be into having his own record as a memento https://www.etsy.com/listing/1256989834/custom-12-inch-vinyl-record-2-sided-40 )
4. If you're looking for something that costs less, could put together a custom mixtape of your favorite classical pieces (or ones recommended by TwoSet). Handmade album art optional
5. This is a longer shot, but if there's a classical music concert that your family is willing to take him to, buying tickets might be cool.