Thanks for your query,
Rash and runny nose can be a part of viral illness and will take few days to settle.
I suggest you to apply calamine
lotion over rashes 3 to 4 times a day also give antiallergic like benadryl
as per dosage.
Watch for any other symptom like fever
or rash spreading to other parts and in that case get him evaluated by pediatrician
Wishing your child good health.