It's probably nothing to worry about - green is on the normal spectrum of baby poo colours.
Oversupply or a forceful letdown can give them symptoms of lactose overload (it's not true intolerence) - the ABA is a good place to get some advice on that. You can always call the helpline to talk to a counsellor. I think the poos tend to be frothy if that's the case.

In any case, it sounds like he's doing very well.