While a huge chunk of the poem shows the boy following his dad around the field while he plows, there are some subtler forms of following going on, too. We see the speaker literally follow his dad, but he also makes it pretty clear that he wants to be like him when he grows up—to follow in his footsteps and take the same path in life, probably to become a skilled farmer like him. While that doesn't seem to happen (maybe he does end up being like the old man in some ways, but he doesn't become a farmer), the end of the poem flips the script: the roles are reversed, and the father now follows the son around.
Lines 10–12: Our speaker has got to be following his dad pretty closely in order to see him squinting carefully at the ground. He's giving his dad the hawk eye in a serious way. He doesn't want to miss a thing.
Line 13: This is a good example of the boy literally following his dad around, and figuratively trying to keep up with him. Literally we can see him bumbling around the chunks of sod and struggling to keep up. But we can also see that he "stumbles" in trying to grow up to be just like his father. He's never the crazy skilled farmer that his dad is, and never will be. He'll always be stumbling in that regard.
Line 17: Oh man, how our speaker would like to be the seed his dad plants that ends up growing into a farmer.
Lines 19–20: Yeah, so it doesn't really work out. Even though he wanted to grow up to be like his dad, he could never really get the farming thing down and lived in the big shadow of his father for a long time.
Lines 22–24: He lived in his shadow for a long time, but these final lines are proof the he finally climbed out of it and found his own patch of sun. Now the father is tagging along in the son's shadow, annoying him in the same way the son used to do.