Michael's too exhausted to throw things and scream. It might happen later. The potential for it is there, he can feel that now. There are all kinds of things in there. But he's so tired. He's so tired that with Lee's reassurance, his body just gives up without his permission and he starts to cry openly.
The hand that's not hanging onto Lee's for dear life briefly comes up to try and cover his face, but Michael can't even concentrate on an effort like that. Instead, he ends up leaning over and pressing his face to Lee's leg. He does want to be close to her. Earlier he didn't so much, but at the moment he thinks that if she stops touching him something painful might happen, like a heart attack or all of his blood vessels bursting.