I don't think I would try bleaching it, but one of those wash-out dyes like Henna shouldn't cause a problem. Does he have open sores on his head from the scaling? You could spot-test it and see how he reacts.
There's nothing wrong with the hair with LI, so if you have any problem, it will likely be the scalp scales absorbing the dye or burning because the skin isn't intact.
Ichthyosis-en-Confetti Type 2
Husband, Nathan - 10, Elliot - 7, Oliver - 4, all affected.
I also have an unaffected daughter, age 8.
email: jennifer at confettiskin dot com
facebook - find me on "ichthyosis mommy spot" or "friends of ichthyosis"