How does the boss react to Lennie's silence in Chapter Two?

In Chapter Two of Steinbeck's novella Of Mice and Men George and Lennie report to the boss of the ranch for work. They are late because George was content to sleep in the clearing next to the Salinas River the night before. Because Lennie is mentally challenged and has trouble thinking for himself George has instructed him to stay quiet while they speak to the boss. George is afraid that if Lennie speaks the boss will not allow them to work. Above all, George wants to make money in order to buy a small farm. When George does all the talking for the two men the boss grows suspicious:

The boss pointed a playful finger at Lennie. "He ain't much of a talker, is he?"

George quickly explains that Lennie is a "hell of a good worker" and "strong as a bull." Lennie temporarily forgets himself and repeats George's words, bringing more questions from the boss which Lennie struggles to answer. When George answers for him, the boss becomes even more suspicious and accuses George of taking Lennie's pay. George assures him that's not the case, but at the end of the interview the boss says,

"But don't you try to put nothing over, Milton. I got my eye on you...I seen wise guys before."

When the two men leave George immediately scolds Lennie for talking and worries that they will eventually get "canned" by the boss. As usual, Lennie is apologetic for his slip.