Castor oil is an excellent moisturizer (but a bit sticky, as oils go). I believe Edgar Cayce thought castor oil had somewhat miraculous healing properties?
If the just plain castor oil does not seem to work well enough for your son, you might try a product made by Alba called "Un-Petroleum Jelly". It looks and feels like Vaseline (so it stays on the lips well), but it is made mainly of castor oil.
I often use it as a lip balm, and it also makes a great moisturizer for sensitive area under and around the eyes.