Skin rash that is itchy is usually due to allergies. A simple rash is an inflammation of the skin (dermatitis). Contact dermatitis is caused by things your skin touches, such as
chemicals ( in elastic, latex, and rubber products), cosmetics, soaps, and detergents, dyes and other chemicals in clothing and plants like poison ivy, oak, or sumac.
If he can identify whether he has had contact with any of these in the recent past, then any one of these could have triggered this rash.
Any medications that he has taken recently or certain foods like fish, eggs and berries can cause an allergic outbreak.
He can take an anti-histamine medication like Benadryl or Cetrizine and apply a 1% Hydrocortisone ointment on the affected areas.
Also application of Calamine or Pramoxine lotion on the rash can alleviate the itchy sensation.
If it is a heat rash, the best treatment for heat rash is to provide a cool environment. Keep the affected area dry. Dusting powder may be used to increase comfort.