I feel for your son, I have experienced outbreaks of hives myself, it can be a real curse. In my case here is what I have noticed and suspected as triggers-
Periods of elevated stress, and food intolerance / allergy (in my case it seemed to be chicken, eggs, and certain amino acids, I did not pinpoint which amino acid or if it is more than one amino acid). However from what I understand, hives can be caused by an abundant number of things, and each individual is different.
But I do agree with the others who suggested MSM and omega 3,6. I have also had good results with grapefruit seed extract, up to 75 drops per day in divided doses, and Colloidal Silver max dose. One other very important thing is sleep. I have had outbreaks when I did not get appropriate rest. It has seemed to have something to do with the immune system. I very rarely have outbreaks any more. Also, I have been doing Liver Flushes every 2 to 3 weeks for the past few months, and have noticed overall improvement in my skin problems.