My 10 Yr Old Son Has Had Petechiae Spots On

My 10 yr old son has had petechiae spots on and off for a few months in random spots. Recently he has been very itchy. His latest rash to appear is on his thigh and is extremely itchy. Doctor apt tomorrow. I am concerned. Should I ask for any specific tests? he happens to get the rash on his left side of body only. Very strange.....