Reactions from the MMR vaccine

Li
My son is 18 months old and is having a reaction from the MMR vaccine. He has had a high fever since Monday. I was wondering if anyone else has had any experience with this and could share what their child experienced. Four days seems like a very long time to have a high fever. The doctor keeps saying it just has to run it's course.
NOT INTERESTED in opinions about vaccines or any argument. I'm a mom asking if any other moms have experienced what my child is going through.