Certainly the main figure of speech that it is incredibly important to grasp to understand the poem as a whole comes in lines 3 and 4 of this excellent poem. Let us analyse these two lines in depth to explore Jonson's own feelings about his son and God:
Seven years thou wert lent to me, and I thee pay,
Exacted by thy fate, on the just day.
This is thus an excellent metaphor in which Jonson compares his son to a loan from God that came due after seven years. Thus we can see that in this poem that is a poem of grief and sadness, Jonson is presenting the "ownership" of his son as not really being his, but being something that has been entrusted to him by God and was never truly "his" in the first place. This central metaphor acts as the base for the rest of the poem where Jonson tries to accept the fact that perhaps he "loved" his son too much.