What Is The Treatment For A Rash In The Leg?

My son, who is nearly 10, has a rash on the top of his inside legs (more on the left leg), the spots are in a cluster of about 15-20 spots, which seem raised and blister like, full of water or puss. They are a little itchy, and some times painful, when his trousers rub on his legs. I just wondered if you had any ideas whaat it could be? Thank you