Thank you for your query on HealthCareMagic.
I presume that the rashes disappear at times and reappear. By your description the rash appears to be urticaria
which is usually an allergic phenomenon. I suggest that you consult the pediatrician
who may get him investigated and treat him accordinly. In the interim you can give him some anti-allergic medication like cetrizine
I hope I have answered your query to your satisfaction.
With best wishes.