What Causes Infected Blister On My Son's Scalp?

My son is a wrestler in high school yesterday after a match he showed me a sore on his scalp like a blister, this morning it spreed to over a 1/4 of his scalp all pussy and red like blisters I called his doctor but of course it is Sunday and theres no none in the office, I called the hospital but they cannot advise me should I take him to ER or wait tell Mondsay to see hos regular doctor? and what do you think it is? thank you