5 month old trouble pooping

Every time my son poops, which is once a day typically, he spits up. It comes out of his mouth and nose and it always chokes him and scares him. I talked to his pediatrician about it and they brushed it off because he still poops every day. But they're always soft poops so I don't understand why he's having so much difficulty with it. I'm really frustrated because it scares him pretty bad and he chokes and cries for about 5-10 minutes after he poops. I've just started him on rice cereal and first foods a few weeks ago and all the info I've found so far simply suggests no bananas and no rice cereal. Any help and suggestions would be greatly appreciated!

-I feed him Gentle Formula because I was not producing enough breastmilk and he lost too much weight while I was breastfeeding. He eats 6-8oz every 2-3 hours and one container of first foods a day. None of the first foods have made it easier or more difficult for him to poop. This started just a few weeks before I started him on rice cereal and first foods.