Hi, Welcome to health care magic. Welts in a child could occur due to a lot of causes, some of them being food, colorants, preservatives, insect bites, infections ( usually a viral cold) and worm infestations. Mosquito bites could cause rashes in a sensitive child, but the picture looks different. They will have smaller red papules instead of welts or wheals as you have mentioned. In many cases, the cause for the wheals cannot be pinpointed. You could give him antihistamines like Benadryl and apply calamine lotion to soothe the rashes. If it is not subsiding or if he is developing swelling around the eyes, lips or difficulty in breathing, please take him to the casualty. Hope this helps you.