He is having contact dermatitis from the vagisil.
He should take an anti-allergic medicine, an antihistamine. He can take Benadryl elixir for children, an OTC anti-allergic medicine. The dose is about 0.5 mg per pound of bodyweight (or 1.25 mg per kilogram). For example, a 25 lb child would get one teaspoon of liquid or one chewable tablet every four to six hours. Follow the directions on the pack.
Apply 1% Hydrocortisone ointment on the rash. Although its use is restricted in children, he can use it as he is above 2. Apply lightly twice daily and for 3 days only.
Also, apply a soothing lotion like calamine lotion, also available OTC.
This should do it. If it persists show his pediatrician.
I assume he used the vagisil accidentally?
Hope I have been able to clarify the situation for you and helped in resolving your query.
If you are happy with my service, your 5-star rating at this time is most welcome.. If you need more help, please let me know so l can continue to assist you.
Thank you and I wish you the best of health - stay safe!
Dr. P. Jyoti