Wayne has been using Apple Music and it seems to use a lot of data. Leo says that T-Mobile has a great gig going on where they allow users to stream for free, and it doesn't count against user bandwidth caps. But AT&T doesn't offer that.
Leo suggests looking in the Music app for a quality setting. If he can turn down the bit rate to save data, then he'll be ahead of the game. Otherwise, downshifting to 3G by turning off LTE when he streams music may be a good idea. The chatroom says that he can cache his music when he's on a Wi-Fi network and then play it without taking a hit on his data plan.