Hello,
Without examination,it is very difficult
to confirm a diagnosis,but it can be due
to heat rash,contact dermatitis
to socks or shoes,rashes due to friction rub or sweat dermatitis,
Wash the areas several times with fresh water. Do not use any cosmetic products at the sites.Also ask your son to avoid wearing leather shoes for some time and wear cotton socks which should be changed atleast twice a day.He can apply some calamine lotion at the rash as it will help in soothing the skin.