Celtic music I have to be very, very careful with. I love it, myself. S is a bit...weird about it. He's also weird about traditional Serbian folk music (not to be confused with gypsy music), though most of that seems to require a brass band. Preferably a tuned brass band.
Anything from BSG would go down fine. S and I chomped through all 4 seasons, plus the miniseries and Razor. The Rohan theme would also be nice.
Actually, Baroque music works with the viola. There maybe isn't such a broad selection of stuff written for viola, but Vivaldi had all that violin music that you can take down a fifth (or sometimes just play straight out, depending on how high it goes) and the Bach cello suites are fantastic on a viola. Kick it up an octave and off you go. And if I get the choice, I shall insist on the baroque bowings.
The Baroque and Romantic periods were my favorites to play from. Baroque is fun and pretty and Romantic music is a good and healthy emotional outlet for angsty teenagers. Even though I often wasn't up to the technical demands (did I ever mention I have no talent for music whatsoever?).