If he has the same age or is younger than you and you are in friendly terms, you can say : 너도 잘 자
Otherwise use his first name John 씨도 잘 자요.
If you are talking to some one whom you have to show respect then you must use honorifics eg
Son : 아버지 안녀히 주무세요
Father: 너도 잘 자라