Welcome and thanks for posting your query to Healthcare Magic.
According to your query your son is suffering from "Chronic Urticaria" and you require a permanent relief.
Urticaria is a common condition caused by bursting of allergy
producing cells known as "mast cells" due to certain aggravating factors present in the environment. As the chemical inside them is released it produces a rash
which you have mentioned. There can be multiple factors producing this condition which are included in day to day life and some times it is really difficult to find one exact cause. In your case you are correlating it with exercises, hot climate,spicy food. So it is better to stay away from the offending factors.
The long term effective treatment for this is by oral anti histamine
under medical advise. It is one of the drug which is not having any side effects and can be taken for long term usage.
Most importantly you should understand that it is a part of good immunity in your son and not a disease entity per say.
As far as the trimming of hair concerned you can avoid trimming if you feel by trimming the baby won't look good. By cutting or not cutting there is no effect on hair growth.
Hope these informations will help.
"With good health wishes"
Dr Sanjay Kumar Kanodia
MD (Dermatology & STDs)