The effort it takes to put pressure on the pencil or crayon may be causing the problem. What happens if you use markers or those soft grip rollerball gel pens? That's what we let our kids use. My husband has several fountain pens because they take less effort to write with. Also, he has typed nearly everything since he was young because it is less difficult and more readable. My husband is blessed with both ichthyosis and being left handed.
These are the pens we like: http://www.pilotpen.us/products/#anchor_g2.
They are sort of expensive at about $1 a piece, but a definite improvement over a Bic or a pencil.
My sons are definitely slower at getting their writing done compared to their classmates, but I have never had problems with them behaving out of sorts like you are describing, so I can't really say much about that.
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"