During the surgery, he moved some of Nat's muscles around to get to the tumour, so he doesn't want to Nat to just lie in one position. He wants him to move around to make sure he heals properly.
Next they're going to turn off the spinal fluid tap to see if he can handle the spinal fluid he's making without any medical intervention.
Nat was was sitting up in bed earlier, and he got up and took a few steps, but he was crying and cranky and wants very much to go home, so they sedated him again.