Suggest Treatment For Red Bumps

my 09 year old son has been getting, one at a time, these very strange looking pimples? It starts with a red bump. looks like a mosquito bite. It does not itch. Within 2 days it is still looking like a bite, but now it is surrounded with redness, size of a grapefruit, it is warm to touch and it is very painful. By day 4 this thing has a pin head sized pimple, and is filled with puss. It is now too painful to even get within 3 inches of this area. After a day at school, the puss has gone, I'm sure due to his clothing and then it takes about 10 more days for it to go. It leaves a purple colored scar just under his skin and feels a bit hard when yopu run your finger over it. I should note that we didi go to a dermatologist and he said he had no idea what this was, however, it being warm to his touch, he put my son on Cefdinir 125mg x 2 daily. I don't think this medicine really did anything, but I could be wrong. I also took Evan, my son, to urgent care when he experienced his first of 7 outbreaks and he said he thought it was a bacterial infection and put him on the Cefdinir at 250mg x 2 daily for 10 days. Also, forgot, as this thing heals it does peel as a blister might. We are off to his MD tomorrow but I'm so worried just need to have some kind of idea of what this could be. My son is about 10 pounds over weight, he is extremely smart and does have a hard time eating fruits, he really has a texture issue, but yes I am aware that I can change this and that will be the first thing I work on, a healthier diet. I do cook ,we are not a fast food, soda drinking family and we do vegetables with dinner every night along with an ice cold glass of milk. Please help, I'm really confused and quite worried at this point. Thank you.