The second half of the poem talks about how to become a poet. Nature is an important part of this discussion, which is unsurprising when you learn about Coleridge's romantic sensibilities. Indeed, the speaker says that Derwent must "delight in the things of earth, water, and skies," among other things, if he wants to become a poet. So according to Coleridge, a love of nature is necessary for poetry.
Lines 11-12: The speaker describes what will happen if Derwent, his son, takes "delight in the things of earth, water, and skies." The Derwent is also, incidentally, a river in northern England, so the kid has got nature built into his very name.
Line 16: The speaker describes Derwent's "father on earth" and his "father above," referring of course to his natural father, Coleridge himself, and God.
Line 17-18: The speaker says that, if his son could stand on the top of "Skiddaw," the fourth tallest mountain in England, he wouldn't see anybody who loves him as much as his father, S.T. Coleridge. Nature really comes in handy, huh?