Well it may be some allergy. Try to find if he was exposed to any new thing in food, or clothes or chemicals etc and stop if u find.
Try local soothening agents like Calamine lotion or aloe Vera gel.
Next Steps
If it continues to increase and there are other symptoms along with like fever vomiting loose motions etc please consult a doctor
Health Tips
Always keep an eye on the hydration of your child and his general activity level