Lots of crying during feeding

Our 15 week baby is crying a lot halfway through a breast feeding. He arches his back and cries as if he's in pain. We can calm him down a little by holding him vertical but as soon as we put him back to breast feed, he starts crying again. We burp him a lot and that doesn't seem to be the issue. He's making 1 big poo a day consistently, unsure if that's the issue.

He also hates being on his back for more than 10-15 minutes and will start to cry, only calming down when we hold him.

He loves to be baby worn and pushed in a stroller but we can't do that all the time.

Any thoughts as to what it might be?