The central message of the poem is that we can either choose to look back and grieve or we can choose to look ahead and imagine. The speaker has grown used to passing the hospital where his father died and to looking up at his father's window. But now, the hospital is being torn down to make way for some new structure; he does not know what.
He thinks about how he would like that new structure to retain some of the "ache" of the hospital, something "unappeasable"—perhaps even death—so that the reality of what was once there remains somehow still present as a testament to his grief. However, when he sees the crane moving "delicately in the sky, in its own language," it almost seems like a pen writing a new possibility that he hadn't ever imagined.
He decides to "forget all that" he wished before, and he prefers to think that the construction will produce "a marvelous house" full of music and joy, something that will "prevail"—perhaps life rather than death—something that will "lift [his] father from [the] bed" and allow the man to climb down the brick "effortlessly" so that he can "run off with his life in his hands."
The house would not contain him or his memory as the hospital once did, and it would represent life and all that makes life good and fulfilling. His father's face will not be "pin[ned] [...] to a window" any longer in the speaker's memory; instead, now he can imagine his father escaping pain and death rather than succumbing to them.