Maybe your pediatrician isn't that supportive of breastfeeding? My pediatrician congratulated me when my daughter had gained 2lbs at her 2wk checkup (6lb 1oz to 8lb 2oz). We did discuss why she might be overfeeding (colic, reflux), but she never suggested that I was doing something wrong or that the weight gain was putting my child at risk.
I've heard that breastfed babies are often fast-gainers and then taper off later. Has your pediatrician taken this into account? Does your pediatrician think there could be a medical reason for your son's rapid gain? If not, you might want to discuss your doctor's recommendations for altering your breastfeeding with someone more knowledgeable about breastfeeding.
I'm a new mom and just trying to figure everything out myself!