Hi I had 湿疹 for a year and gained a lot experience to fight it. Now it's almost gone.
my advice is go to a dermatologist for a new 激素类的膏药 (never used before) and use them twice a day for a week. In the whole week, try not to take any bath. and avoid water on affected areas. Also in the whole week, your son should stay at home and try to wear shorts so that no clothes can touch on the skin. Good luck