O thou, my lovely boy, who in thy power
Dost hold Time's fickle glass, his sickle hour;
Who hast by waning grown, and therein show'st
Thy lovers withering, as thy sweet self grow'st.
If Nature, sovereign mistress over wrack,
As thou goest onwards still will pluck thee back,
“Who” refers back to the “ lovely boy ” of the first line. The  metonymic  image of the moon continues  the reference in line two; the sickle-shaped moon is “ waning ,” that is decreasing in size. At this time, this was language commonly used to represent a decrease in power or a decline. Interestingly, the youth has paradoxically increased,  “grown” spiritually, or somehow improved with the passage of time. 
 Compare to the first line of  Sonnet 11 , where “waning” or mortality can be counteracted by the birth of a child. By Sonnet 126, the speaker has long since given up convincing the fair youth to procreate. 
 
 A moon waxing, then waning.
Sonnet 126