He's a Yorkshire TERRIER. He's completely normal. At only five months of age, he will never tire of playing!
He will eventually calm down some so that he will gladly sit or sleep in your lap, but it will be awhile. In the meantime, playing and interacting with you is helping him to develop into a well-rounded puppy both physically and psychologically.
I almost forgot. Nice, loooooong walks help a lot to wear them down, too.