Hi WorriedMom - sorry you are having to deal with this.
Take a picture of the bite/rash and take him to the Dr.
Ask for antibiotics - he will need to take them until he is symptom free for 4 weeks.
An EM rash is Lyme Disease.
You can also schedule an appt with a LLMD in case it doesn't resolve and you need someone more knowledgeable to treat him. Explain to the receptionist that it is a new bite - she may be able to squeeze your son in sooner than later.
In addition to recommendations from members here, you can also email me for LLMD names.
Here are the ILADS Lyme Treatment Guidelines. Scroll down to page 18 for antibiotics and doses. You can also see the treatment categories showing how long to treat.
Maybe show them to your sons Dr.
www.lymenet.org/BurrGuide200810.pdf