In the past, the father has tried to help the child cope with earache by showing him how the ear works, providing a visual image of the ear's anatomy from the encyclopedia. It didn't help the pain. Now that the father is chronically ill, his adult son tries to help, but cannot (even with an encyclopedia that purports to contain the entire world) produce a visual image, or metaphor, to explain the illness, or "what brought us here." Instead, he gives his father a Walkman, hoping that the music will "help him pass the hours in dialysis."
The son can see his father's blood, circulating in the dialysis machine (just as the father helped the son visualize what was happening in his sore ear), but the son cannot feel (or see, or hear--or, even though he's a poet, find a metaphor to convey) what it is like to be his father. This isolation is ironically emphasized by the son's gift: because of the Walkman, the father falls asleep "to a music I can't hear / And for which there is no metaphor."
