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probably its a urticarial rash i.e. allergic rash
.We also have to rule out scabies since he is a child .Kindly look if he has some lesions on his web spaces..is the itching more in night..Does he have any stomach ache or any dental caries
.You can de worm him by tab zentel...give his antihistaminics like cetrizine
(5mg) daily..Show him to a dermatologist
who can see the lesions and diagnose.