Thank you for your query.
As you have not mentioned the age of your son it is difficult to give a proper guidance. A number of the viral upper respiratory infections follow with a rash
. Most probably your son is having them. As he is back to his normal behaviour you can relax as the rash shall subside on it own. In case it persists then you can follow it up with your doctor.
I hope I have responded to your query to your satisfaction.
Wishing your son all the best.