9mo old dropping in weight %ile?!?

My baby was born at 8lbs (50ish %ile) and by 2 weeks weighed 8lbs10oz and didn't stop the rapid gain there. He was 14lbs2oz (97%ile) by 2mo, 17lbs6oz (95%ile) at 4mo, and 19lb13oz (83%ile) at 6mo. 

Fast forward to now; he is 9mo and 20lbs even (40%ile). He's been in size 4 diapers and 9M clothes since he was 5 months. He is my chunky monkey still and his rolls aren't going anywhere. His height has stayed around the same %ile, however his weight isn't budging. I wasn't concerned until my ped suggested possibly supplementing and insinuated I may not be producing enough. I know this is not the case because I hear my son swallowing and have a strong letdown still and he still nurses 5-6 times a day along with 2-3 times overnight. Plus 1-2 solid meals a day. 

I also want to add that he is one of the most active babies I've ever seen. He if he's not crawling across the entire house, he pulling himself up on furniture, standing and playing on his push walker, chasing our dog, climbing up my leg, etc. I think this is a key factor is his lack of weight gain. 

I REALLY want to nurse him full time until 1 year and hold on to bedtime nursing until about 18mos-2yrs. So, my ped's advice to supplement with formula in a cup was very discouraging. I have very little support to continue nursing in real life. So hearing stories of mom's who are or have been in similar situations will be SO SO SO helpful! 

TL;DR - My baby stopped gaining weight around 6mos due to activity level and ped wants me to supplement with formula. Need words of encouragement!