fattening up your baby

I've been off the app for a few months. But I need advice! My boy was born 6 lbs 14 oz and has struggled gaining weight coming in at 7 months only 14.5 lbs. In the past three months he has dropped from his curve and is now in the bottom 1 percentile for his weight. Pediatrician has been having us come in monthly for weigh ins. No allergies, no health problems, we feed on demand. I pump non stop, always have milk leftover, feeding solids 2-3 times a day. But the kid won't gain! Last month he only gained 6 oz! And that weigh in was after having not pooped for three days. Pediatrician wants us to start doing one bottle of formula a day to see if that will help. Any advice to plump my kid up? Here's a picture that makes him look like he's round.