So, you know how after Michael got scooped, he basically didn't have any insides anymore?
I like to think that after Ennard left him to rot, Michael wasn't able to walk or stand at all due to this.
Now, getting to the point, after Ennard left Michael, Michael decided to go back into the sister location, and gather wires and parts from what was left of the decommissioned animatronics. He used the scraps to create an endoskeleton inside of his body, as a substitute for an actual skeleton.
Boom. Now he can move.
Somewhat.
I mean, he is still missing a lot of things that a normal human need to move properly and look normal, but it will do.