It is very hard to deal with an older child if you are suffering from PND or birth trauma.
But you cannot lay that on a 6-year old. It is your responsibility as an adult to get treatment if you need it and if you are not well enough to organise it, it is the responsibility of your dh to support you to do that.
The situation of what to do with the older child when breastfeeding is one that arises in every family that has more than one child. Dd was younger than your dss, so less easy to reason with and far more violent (she would deliberately launch herself on ds when he was at the boob and try to pull at his arms or deliberately throw things at his head). It was not easy. Even so, it was my responsibility to manage the situation so that she still felt loved and cherished. We managed by inventing a game that we all "played" together.
The fact that he is your dss and not your ds1 makes no difference to his needs. If you are the adult there and in charge, then he needs the same emotional support from you that your dd will need when she is 6- plus possibly extra for the upheaval in his life.
Most parents of more than one child have a plan for how they will engage the older child when breastfeeding the younger. A younger child can be encouraged to "breastfeed" a doll, an older child can be read aloud to or sung to.
In your case, maybe your dss could read aloud to you? Maybe present it as something he could do to help you? Or you could have a special story tape or something that the two of you listen to while you are feeding? If he can't fit onto the sofa, can he have a special chair or cushion next to you?
The more you manage it pro-actively, the easier it will get, not least for you.