The modern man in the poem "The Man with a Hoe" is the exploited worker who toils endlessly and becomes degraded so that a few people at the top of the social order can become very wealthy. The poem appeared in 1898 and created a sensation because large numbers of Americans had become concerned with the wide economic divide in that time period.

The poem is ekphrastic, meaning it is based on a work of art—in this case, Millais' painting of the same title. The modern man in the poem is literally a description of the man in the painting. As the poem describes, the man is leaning wearily over his hoe, dehumanized by his ceaseless labor. His soul has been emptied, and he is so overworked that he cannot experience either joy or despair. The poem asks if he is more ox or human, criticizing a culture in which any person is forced to toil so relentlessly.

The poem condemns the "masters, lord, and rulers" for so demeaning people made in the image of God, asking:

O masters, lords and rulers in all lands,
Is this the handiwork you give to God,
This monstrous thing distorted and soul-quenched?

Modern man, according the poem, is made for better and higher purposes, no matter who he may be.

Edwin Markham was an American poet, and "The Man With The Hoe" was first published in 1899. The eponymous man is thus most likely a personification of the American working class at this time.

The lives of the American working class by the end of the nineteenth century were undergoing drastic and adverse changes, largely because of the Industrial Revolution. Machines were being invented that could perform tasks more quickly and at more cheaply—the same jobs that required manual labor previously. There were, therefore, fewer jobs available and more unemployed workers looking for employment. This also inevitably led to lower wages for those who could find employment.

In "The Man With The Hoe," Markham draws attention to the condition of the average working-class man and criticizes the pressures, such as those consequent of the Industrial Revolution, which reduced this man to "a brother of the ox" and a "monstrous thing distorted and soul-quenched." From his poem, we might infer that the average working-class man in America at this time was "Bowed" beneath the weight of his own poverty and was made a "Slave of the wheel of labor" by the "blind greed" of the world.

I am assuming that you are asking about Edwin Markham's poem, which is based on a painting with the same title.  The man in the poem is clearly meant to be a man for all time, representing ancient workers, workers at the time of the publication of the poem, which was in the late 1890s, and workers to this day.  

In the modern world, there are people all over the world, men and women, who continue to carry "the burden of the world" (line 4), who are so exhausted by their labor they cannot even feel "rapture or despair" (line 5), in other words, great joy or sadness.  These are people who are "Plundered, profaned, and disinherited" (line 30), so taken advantage of by the greedy that they have no lives, no spirit, and no minds, little more than animals because of how they are treated.

In spite of the fact that in some countries, there are some protections for workers, due to enlightened management, legislation, or union efforts, the plight of the working man and woman persists.  There are countries where working people make a few dollars a day and are subject to dreadful and dangerous working conditions.  They have been stripped of their humanity, too exhausted to go home at the end of the day and enjoy anything life might have to offer. Even in the United States, many people have dangerous and dirty jobs that afford them not even a living wage. And even for those who make a living wage, for example, coal miners, the weight of the world, which is literally upon them, bows them down. Migrant workers are a group of people who  endure backbreaking work, as well, and it is likely one can find, right now, a man with a hoe who is the very kind of person Markham is speaking of.  The fact that we are living in a post-industrial age has not made Markham's man with a hoe disappear. Lamentably, he is still with us.