Is your doctor aware that pump output is not indicative of overall milk production OR what baby takes in at a feeding (since baby controls his own intake)?
Remember that weight charts just plot a growth curve. Percentiles are averages. Someone has to be at the top of the chart and someone has to be at the very bottom. Weight is only ONE indicator of baby's overall health. It's important to look at the whole picture (and by your own account, your baby is doing well).
Here's a (non-LLL) resource that you might find helpful and reassuring: