Why do you think the double bass allows him to play beautiful music this one time?

The double bass seems to pay the boy back for his interest and respect. He has never bothered practicing much, so cannot play very well himself. Clearly something magical happens when he does manage to play such wonderful music. This seems like the instrument has saved him from an embarrassing situation in which his lack of skill would become apparent. The music perhaps represents his passion and innocence, which the instrument has picked up on and feels a joy in. However, it only happens once because the boy drops the instrument and it no longer feels cared for or respected. The boy moves on to more grown up concerns and leaves the instrument forgotten.