We begin with good-bye. The speaker is saying farewell to his son after only seven years. Sad. It seems that the speaker blames himself in a way for the loss of his son.
The speaker then envies his son, who (since he's dead) is free from both the physical and the mental pains of life. The son also won't have to worry about the hassles of getting old, either.
The speaker then asks his son to tell anyone who asks that he (the son) is Ben Jonson's best piece of poetry. Then, for his own sake, the speaker vows in future not to like the things he loves too much.