Melissa is absolutely correct! Your ped must have very limited knowledge of breastfed babies. It is correct that he is basing the percentile on a formula fed chart. Formula fed babies bulk up and are often overweight because of the high fat content used in formula. Its made from cow's milk, which is designed for a cow...not a human! Baby cow's need to fatten up quickly. Breastfed babies have balanced nutrition that is appropriate for humans. The correct charts can be found here.
http://www.who.int/childgrowth/standards/cht_wfa_boys_p_6...
Remember that this is in kg, so the conversion would be done here:
http://www.convertunits.com/from/kg/to/lb
Example: A 9 month old who weighs in the 50th percentile would be 8kg or 17.5 lbs.
Formula is not the answer. Keep nursing, let him nurse as much as he wants. Breastmilk can and will sustain him and it is enough...no baby NEEDS formula. And who said being below 50% is an issue anyway? Some babies are just average...do they have to be HUGE to be considered "healthy?"
You also have to keep in mind (as Melissa also said) that babies will lose somewhere between their 9 month and 12 month visit. This is perfectly normal as they are more active and getting more distracted while eating. My ped was not at all concerned when my baby lost weight and dropped his percentile...she said if he didnt begin gaining by his 15 month, then we'd have to talk. But guess what? He gained by then and all was fine.
I'm sorry if I seem a little snappy. It isnt you...I just get heated when doc's give out crappy breastfeeding advice. Do you know in medical school they arent required to take a lactation course? There is an optional 1 HOUR course...OPTIONAL! Yet they think they are qualified to help nursing Moms...too much bad info out there. And let me tell ya...I'm a breastfeeding counselor. So when I give classes to expecting parents (and this is just covering the basics to get them started) it takes almost 3 times as long as their "optional" course! Phooey! See an IBCLC if you want breastfeeding advice. Not a ped!
Good luck to you. You are doing the right thing. Trust yourself to nourish and provide for your baby! If you need any assistance (boosting supply or other breastfeeding issues) please feel free to contact me. If you are truly concerned about his weight, try giving him some healthy high fat foods. Things like avacado, olive oil, etc... Or you can just eat them yourself! Eggs (for you, not him yet), Ben and Jerry's ice cream, (not exactly healthy, but sure is yummy!) Hang in there Momma...you're doing great!