How Can I Fix My Portable Compact Disk Player Earphone Problem?

Hello:

My son has a SONY compact disk walkman-D-EJ360. Only one earphone side now works. Instead of stereo, it now plays in mono. Other than the earphone the CD player works fine. Can I repair this or is there a earphone set one can purchase to correct the problem. I notice that when I set the earphone jack in the CD only halfway, the earphone produces the normal stereo sound. The CD player is one year old.

Any advice would be appreicate.