Totally normal unless there is blood in his stools.
Green stools are not always cause for concern, but CAN be allergy related...dairy being the most common culprit even the hidden dairy like whey, casien, lacto anything, all nuts, all seafood. or a green or blue food eaten. Green, blue, or yellow artificial colorings too.
Stool starts out green , then turns bright yellow as it goes through the digestive tract. It is bile and bacteria that finally turn it brown. Yellow or green stools can indicate stool is passing through the digestive tract too rapidly not giving it a chance to change colour.
Bright yellow stools can also indicate not enough bile / billiary obstruction.