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This condition is called URTICARIA
OR symptomatic dermographism...Which means that either he is allergic to something or it a immunological problem.Also any kind of infection in the body can provoke this kind of symptoms...so watch out for any infection in body,like tooth decay or worms in the stomach or any fungal infection
in body.You can start with deworming him first and then look for any such infection.Rarely it can be due to a thyroid
problem also.If it is due to any allergy
,like to any food ,then that needs to be found out.There are allergic testing done in certain labs where you can get this done.I would suggest you to meet a good dermatologist
for complete work up.