It could well be because of your forceful letdown. Have you tried expressing a bit before attaching him? Does the poo look frothy as well?

Is he generally a settled baby? If it were me I would be investigating further, as there can be a number of things causing green poo in a baby, but maybe I'm a panic merchant! Not that I think there's anything terribly wrong with your baby, far from it, but if a mum feels that she wants something checked, then she's probably onto something. Do you have a paed?