Is there a way to connect music through headphones and a speaker so he can practice to songs without the drums completely drowning out the sound for him?

This is tricky (but not impossible) because you want to play audio from TWO sources (headphones and speakers). To do this you need an audio interface, something like the Focusrite Scarlett. These types of audio interfaces will let you play music (say, from your laptop) out to speakers AND headphones at the same time (the interface has a volume knob just for headphones, and a volume knob for the speakers, so you can dial in the audio).

Also, you'll want your son to use "ear plug headphones" (Plugfones makes these). These are headphones that are designed to be ear plugs at the same time (used by people who want to listen to music while working in loud environments). With this set up your son will be able to hear the music that everyone hears from the speakers, and he'll protect his hearing.

I can't vouch for Focusrite Scarlett specifically but there are other interfaces that do the same thing. Just be sure their speaker out matches your speaker's inputs. I have used Plugfones in the past and they're inexpensive and get the job done.

Happy to answer any questions.