First question i would have asked from you would be is there a rash
anywhere else on the body?
iF NOT then i would suggest that its a simple case of Contact Allergic Eczema
I think he needs to apply 1% Hydrocortisone
ointment once a day for ten days
10 mg for 10 days in the morning.
3: Observe the rash ,if the above medications dont help in 3-5 days then you need to see your GP or dermatologist
i think i can not help more than that with such a brief history so please bear with that,thankyou and all the best.