ok to stop breastfeeding cold turkey?

KT
He's on formula to but can I just do formula or if that to much for his belly? He's 3 weeks and 4 days old. We did Enfamil gentle ease for 5 days he didn't handle it well. Today we did similac total comfort and besides not sleeping to long or the spitting up he's better. Can I just do formula or is that to much on his belly?