Why Does My Newborn Baby, Who Is Both Breastfed And Fed Nan Pro 1, Have Green Stool And Gas For Over 2 Weeks?

Hi. My baby is now 2 months old. He does green stool since more than 2 weeks. I breastfeed him but my milk is not enough for him. So I also feed nan pro 1. Why does he have green stool. He do 1 stool in one day that too green. Please guide me through this. He also have gases.... I am very worried about his motion.