Thank you for your query.
The description that you provide suggests that your son probably has urticaria
). Urticaria is an allergic reaction
but does not occur due to cast or metal in the arm. It may be to the medicines or some other thing in your son's environment which needs to be looked into.
You can give your son tab. Phenargan 1/2 tab. three times a day and see the response. It is always better to consult your doctor and proceed further.
I hope I have responded to your query to your satisfaction.
Wishing your son a speedy recovery.