The two main subjects of Ben Jonson's "On My First Son" are his son and death.
Jonson's title indicates that the poem is about his first son, and the reader of the poem realizes in the very first word, "farewell," that the son has left his father. This could indicate perhaps that he has gone away on a trip, but as the poem progresses the reader comes to realize that the son has died. He wishes his young son to "rest in soft peace" and indicates that this child was Ben Jonson's "best piece of poetry."
This poem is about an unexpected death. Ben Jonson wrote this poem as an elegy for his oldest son Benjamin who died in 1603 at the age of seven. Jonson attempts to reason about why his son had to die so young and can only conclude that perhaps it was because as a father he committed the "sin...of too much hope of thee," making too many grand plans for his future.