Thanks for your query,
Fever associated with rashes is mostly viral in nature,and takes few days to settle.
I suggest you to give him paracetamol
every 6 hourly till fever is above 100F,
lotion 2 to 3 times a day,
give some antiallergic like benadryl
as per dosage.
Give plenty of fluids.
In case symptoms persist get him evaluated by pediatrician
Wishing your child good health.