This is most likely atopic dermatitis, often also called eczema. Scratching it aggravtes and prolongs the duration of the rash. It also potentially introduces bacteria into the skin and result in an infection.
Taking antihistamines helps to relieve him of the itch and reduce the scratching.
Using moisturiser helps with repairing the skin and making it stronger and less prone to breaking, which further aggravates it.
He may need some steroid cream or one combined with and antibiotic as well to reduce the rash further.
A skin scrape will help tell if is fungal in origin so that the most appropriate cream can be used.
Avoid taking hot baths or showers. Whilst doing this might help with the itch initially, the increased blood flow to the skin will make the rash worse afterwards. Try putting a wet towel in the fridge. When it itches, place the cold wet towel on the rash to relieve the rash.
Understand that stress can also trigger the rash and make it worse.