I'm no child psychiatry expert, but it seems to me like introducing a new parent at age 11 would be pretty tough for him to really accept? I would more expect her be, like, an Aunt, someone who should be listened to and respected, but not a third parent.
Me, 30ish bi female, been doing solo poly for roughly 5 years. Gia, Clay, and Pike, my partners. Davis, ex/friend/"it's complicated." Eric, Gia's husband. Bee, Gia and Eric's toddler.