How should I calm this child down?And if he has a disorder what is it?!

Only discuss this with the parents of the child.
They have the right to know of this activity, expecially if it appears frequently.
If he has a disorder, then the parents and doctors can advise you, as the babysitter, on what to do in order to deal with this child.

You don't say how old this boy is.
You don't say if this is normal for him and his parents deal with this, too.

The standard way of dealing with this in a preschooler is to just walk away and leave the room until he recovers. The thinking behind this is that if you don't give him an audience, he may stop this inappropriate behavior sooner because there is no one to impress with it and bad behavior ultimately leads to isolation for him.

I would not baby sit this child again, he could be hyper active or abused child