I am very relieved to hear Ri made it through the night. It's amazing how resilient they can be. I hope he'll be ok.
I found this illustration of a bird's skeleton showing the position of the Synsacrum.
The vertebral column consists of vertebrae, and is divided into three sections: cervical (13-16) (neck), Synsacrum (fused vertebrae of the back, also fused to the hips (pelvis), and pygostyle (tail).
Is it a hairline or total fracture. That spot looks like a big bone on the back.
Cranial would mean head end so you're right in front of. So its in front of the big bone which looks more like a hip. Not sure if that's good or bad but he's alive so theres a lot of hope. Here's hoping for best