Why?

She saw that her son was in pain from the heroin withdrawal. As she held him he kind of started sucking her breast as if he was a baby. It was actually the son who places his moms hand on his dick. Strange scene, from a strange film. But I think the kid was only looking to be comforted by a mother that had always been so distant from him. In the very beginning of the film you see him as a baby, crying while his self absorbed mother ignores him and just continues dancing and laughing. It seems he never got his mothers breast as a baby, or even her attention. I really hated that character portrayed by Clayburgh. Everything was about her, even her sons heroin addiction was used as an excuse for the mom to talk about her career. Good film though.