I agree with #2 - this novel really tells the tale of a struggle of an orphan boy who is trying to find his place in his world at large and has to overcome a number of barriers to do so. Therefore there is no one particular character who plays the role of antagonist as in other novels, rather this role is played by the rather amorphous figure of society, specifically society with all its woes during the Depression in the 30s.
This is a story about a young man coming of age during the Depression. Like many kids at the time, Bud is the victim of the time he lives in. Many parents died young or gave up their children due to extreme poverty. The orphanages were overflowing and foster homes were a tough spot to be placed, as few families had much money. See what Bud says about the last home he stays at:
I watched them walking away. My pretend brother looked back at me and stuck out his tongue, then reached up and took my pretend mother's hand. I couldn't really blame him. I don't think I'd be real happy about sharing my brown sugar and my folks with any strange kids either.
Bud is trying to overcome all of this and is seeking to find a parent, his father. His hope is that by finding his father, he will be able to have a place of safety and comfort.
Bud's antagonist in this story is society. The society of the Depression, full of poverty and crime, is the force that causes him pain. He did not ask to be born into this difficult world, and he is on his journey to overcome the conditions he has experienced. The great lesson readers are left with, however, is that no matter how much society antagonizes you, only you can decide if it will beat you.
Bud's quote (above) was not related to the last place he stayed. This quote is in reference to the kind strangers who pretended to be his family, so that he could get a free meal at the mission, in which he was too late. However, it is a good example of the condition of which most families were facing.