Before I answer that question, let's review the definition of "tone".
Tone is a literary device that indicates the emotion the author intends to communicate in his/her work, conveyed through word choice, syntax, and figurative language.
In "On my First Son" by Ben Jonson, the tone ranges from sadness to confusion to melancholy. He begins the poem by bidding farewell to his son and to happiness: "thou child of my right and, and joy." As he reveals, his son has died in his childhood at the age of seven, for only "seven years tho' wert lent to me."
In line 5, Jonson's punctuation abruptly shifts to incorporate an exclamation, in which he cries out in anger and despair, "O, could I lose all father now!" With his son gone, he is no longer the father he once was, and his identity has also been affected by this loss.
In the same line, Jonson begins to barrage himself with questions, asking himself why "man lament the state he should envy." Jonson goes on to discuss the many disadvantages of life that are avoided by an early death, implying that he finds his own sadness inappropriate to the situation and should instead feel pleased for his son.
Finally, Jonson reverts to melancholy, noting that his son was his best piece of work, thereby tying his career into the death of his son as well.