Neosure Users!

My son was born at 33 weeks and 3 days. While in the NICU, they were adding Neosure to my breastmilk to up the calories. He never had an issue pooping. I am not able to pump even half of what he eats in a day anymore so we have been giving him formula (24 calorie recipe) bottles. It seems like since then he has been constipated on and off. Has anyone else experienced constipation with Neosure? If so, what did you do to prevent/stop it? Thank you!!