My baby boy is a big eater, had to give up breast feeding because I wasn't producing enough to keep up with him

(broke my heart). Anyway he was on 200mls Karicare gold, having up to 8 bottles a day at 10 weeks!!
We were told to use Nuture Plus as it's casein dominant and stays in his tummy longer, so we did and he seems to like it but now his stools are becoming harder and green (Sorry TMI) Should I seek medical attention? or is that normal when changing formula? He seems happy enough atm. Or is there another formula that may be better for him?