My immediate, knee-jerk reaction is to focus on DS's feelings & not worry about the 6-day thing right now.
My *personal* view is that when my child expresses a need & is doing so respectfully, within her "rights", then it is my job to back her up as her Guardian. To me, she's trying to navigate the situation & needs me to advocate for her while giving her the room to make her own decisions - my involvement is simply to provide a stop-gap - a safe zone - for her to lean on while she expresses her opinions & feelings & to make sure that she's being Heard.
If she was doing it simply to hurt my ex in retaliation, we'd start down an entirely different road of conversations.
It sounds really legitimate to me considering their history - it's unreasonable to expect the kid to just jump up & run off giggling like a preschooler. Have you tried talking to both boys together, at the same time? Does hearing his younger brother's opinions help DS16 to open up more?