Adam Bojelian, 12, has cerebral palsy, is blind, and suffers from a severe seizure disorder.
He communicates by blinking and used his skill to pen the song which will help the Children's Hospice Association Scotland.
McGregor said: "It's quite incredible to think that Adam shares his vivid imagination with us through poetry written by blinking."
Adam, from Edinburgh, wrote the words to Christmas Gibbons two years ago. It explains how Santa is able to deliver so many present in one night.
It has now been set to music, recorded, and is now ready for download from iTunes.