What Causes Welts At The Site Of Mosquito Bite In A 15 Month Old?

My son is almost 15 months old, around 30 inches and around 22 lbs and was bitten 2 weeks ago by 2 mosquitos and the raised as they normally would and went away. Well the 24th of this month he woke up from his nap and where the bite areas where, he had large welts. Took him to the doctor and they said he had some sort of skin infection and the put him on antibiotics. Those are clearing now. Now, he woke up this morning and he had a very large welt on the top of his head with 2 pin head sized pustule(sp) sacks and 2 more forming. Please tell me what is wrong with my baby. The doctor that I took him to did no know anything of what he was doing. He told me that he didn t know what my son has and that he would have his wife look at it. Well she did and she told me that she didn t know but it needed to have a culture done because it was pustule(sp). Well he got everything ready and and started to wipe the area off before culturing. Well he wiped to hard and busted the pustule sacks, and then tried to say there was nothing there to culture. Please tell me what is wrong with my baby and what can I do. Thank you so much for your time.