Papular urticaria occurs only after a bite from an insect especially mosquitoes. To prevent new lesions you need to prevent insect bites... especially mosquito bites... by using odomos cream or making your boy wear full sleeved clothes especially when he is playing or sleeping or using mosquito nets. Even a single mosquito can cause trouble. So precautionary measures will help you in the long run. The urticaria should subside in the next 15 days. The marks will take a little longer but will go away completely. But prevention is the key to long term remission of the urticaria.