Thanks for the query,
Probably he has developed an allergic reaction
called as urticaria
Apply a mild steroid lotion like desonide,
Also give him an antihistamine tablet like cetrizine
This will give you him some relief.
Please meet a dermatologist
for exact diagnosis.
Let me know if you have any other doubt.
you can ask a direct question to me on this forum, following the below link.
Wishing you a good health.