Mommies, can you tell me how to fix the stool for a child on SVs? I'm …

Mommies, can you tell me how to fix the stool for a child on SVs? I'm feeding him 60ml of Nutrilon Comfort formula at night. He poops once a day or every other day. He wants more but can't. He gets constipated, worried, and cries. I force him to drink water, he doesn't want it, throws up. At first there was no such reaction with the mixture. In one day he went from 3-4 times to one stool.