A musician with a loss of hearing---no wonder Beethoven went mad! Hearing/listening and music go hand in hand.
Listening to music though, really listening is very important to the budding musician. Encourage your child to listen to not only what he is playing, but how it sounds! If you have some kind of recording device, record his practice and then have him listen to it and describe what he hears. Does he think his beat is steady? Can he tell what song he is playing? If he sings while he plays, ask him if he thinks his pitch matches what he is playing. If you are really bold, record yourself singing along with his playing and ask him if your pitch matches his playing.
A listening challenge to the parents would be for you to truly listen to your child's playing. Close your eyes, and enjoy listening to their progress!