I can sympathise, Nicholas had a strong preference for the right side of his head for months. Turning his bed around really helped us, but it's obviously not helping you!

Have you tried a sleep positioner? You put him on his left, and it will tilt his head to the left. I do it with two rolled up nappies, one on either side of him. But you can get the real thing from a baby shop. It looks awful for them at first, but it will help.

Lots of tummy time too, and when he's on his back dangling toys to his left.