Green poop EBF

Kelsay
My son will be 3 weeks old tomorrow and the past few days has had green poop and has been more fussy than usual. I've read that it's because he's getting too much foremilk and not enough of the high fat hind milk and is most likely because I have an over supply. He is gaining weight, but he is so gassy and his stomach is audibly gurgling all the time. He also spits up a few times a day, and coughs occasionally when nursing. I nurse reclined to help with this.
 I had mastisis when he was a week old and was on antibiotics for a week. Originally I was alternating breasts at each feeding and he had normal yellow stools. But then I was told that can decrease your supply and I'm so worried of having an under supply that I went to giving him two breasts at a feeding. I alternate sides that a start on, let him nurse until he's done on one side, burp and offer the other. But clearly this doesn't work for us? Should I go back to "block feeding" for a week? Has anyone else had any experience with this? What worked for you?