Hi, also not wanting to second guess your doctor, but just wanted to mention that here (UK) the growth charts are based on formula fed babies, who tend to have a different growth pattern from breast fed babies. I think the same applies to the charts used in the US. My ds went from 91% at 10 weeks, to just over 50% at 17 weeks, and my health visitor and GP said not to worry, it was extremely common for breast fed babies. If your ds is otherwise well, energetic, wetting nappies etc I wouldn't worry too much.
Having said that, I agree with Debra's suggestions, also, do you feed on both sides? I single side fed for the first few months, but in the last few weeks have been double sided feeding, and ds is growing more quickly again. Also, you could try and increase your supply by pumping after each feed for a few days.
HTH