Zinc deficiency can definitely cause skin problems in folks with CF! I would try a zinc supplement to see what effect that might have on the psoriasis, in addition to a topical zinc cream. (I haven't heard of CF causing nervous ticks, though!)

As to the itchy feet, I am clueless--we've never experienced that problem. Of course, when CF kids take long baths, they experience tremendous and abnormal skin shedding on the palms of the hands and the soles of the feet. Does your son take a bath each night before bed?