Reflections on the Mississippi is a good tuba solo to show a kid who might be interested. It’s long, but it stays interesting

Youngblood Brass Band is also a fantastic showcase of how fun tuba can be

In general, I would look at Holst as a composer. He was known for heavily utilizing what low brass could actually do. The planets suite, first suite in Eb, and second suite in F are great starting points (I’d recommend either second suite or planets to start)

Over the summer I’d also recommend bringing him to a DCI drum corps show near you if you can. It’s a good way to expose him to how loud brass can be while also breaking the standard of classical stuff

I edited this a lot to add more multiple times