I'd take him back to the doc ASAP!

I would keep him home and take him to the doc again. When you call for the appt, let them know it looks worse and that you suspect MRSA. Even though his helmet gets lysoled frequently, locker rooms are crawling with germs. i.e. Tom Brady and Peyton Manning's knees.

I would insist that a blood test be done to determine if there is an infection and if so, what type. If there's any question you might then ask for a referral to an infectious diease specialist. I'm sure your boy will be fine.

I'm actually a dermatologist in my other life. First statement: you can't make a diagnosis or treat patients over the internet. He needs to see a doctor, today preferably. I don't think a GW Kitchen forum, no matter how great, is the place to get advice!

Best thing which is so often neglected would have been a swab culture from the area at the initial visit, before treatment was initiated, for both bacteria, and also herpes simplex, if they really suspected "cold sores," though that's quite rare in that location. Many do what you doc did, which is to treat empirically with Bactrim, a drug that covers probably 98% of all staph including MRSA. Failure to respond certainly could indicate that this was not staph, or the incredibly rare case of Bactrim resistant staph. Equally likely, as we see this all the time, is that he's doing fine and that what you interpret as "not better" actually is improved to an expert's eye.

Because of your concern, and even more so if other symptoms are present (fever, chills, red streaking, swollen lymph nodes, pus draining, etc etc) a recheck is indicated and I woudl press to get bacterial and HSV cultures if they think it's a consideration. Maybe you can guilt your derm into seeing him if he's worse.

PS: staph is almost certainly acquired at school, not in the temp kitchen.

Thank you all for your response. The ped. did not swab the area because he did not want to drain the blisters and risk it spreading.
clinresga, thank you for your expertise. I would never dream of diagnosing via the internet!! I think I was looking more for support and a "where would you go from here" from mother to mother. My husband, who is actually a public health official here is DC happens to be out of town (isn't that always the case when these things spring up:)
I get cold sores as well, but by day 5 they have usually started to scab over. I am concerned that there are bigger and newer blisters. Of course, if it is cold sore, that is viral and won't respond to the bactrim and the cold sore could have spread....although I think it odd that it is on the ear!!

Thanks all. I am calling the ped back this morning,

K