As you feel him start to underhook your legs, take control of the sleeves and PULL. Same time, flatten yourself out, walk back on your shoulders and drop your legs to the ground hard.
Hook your insteps under his thighs like butterflying over his arms. Pull his head down and flatten him out.
If that fails, shoulder roll backwards and bring the bottom knee through the instant you're turtled. Now you're back where you started.