Hiya
The odd green poo is ok - Ky had a few green poos before he was weaned. I think its safe to say that you can expect green stools every now and then without too much concern.
If your Olivers stools are nearly always green and if he is not gaining weight well even though your milk supply seems fine, then it could be that he is getting too much foremilk and not enough hindmilk. As the foremilk is more dilute, with more lactose and less calories, it stimulates your baby's digestive tract to move the milk along too fast producing green pooh.
Consistently green stools could also be a warning that your baby is sensitive to something, for example any medication he may be taking. If not, could he be sensitive to medications you are taking, like iron supplements? Very occasionally, a baby can become sensitive to something in your diet which is passed to him through your breastmilk. In this case other symptoms, such as eczema or a rash, may be present.
Hope this helps