The shake’s as old as he is
Paterson describes his son's fragility in more detail in stanza three: "The shake" explains the reason he must "steady" his brush in stanza one and why he messes up the circle. It suggests that like the drawing, Jamie himself is flawed. Perhaps Paterson sees his son as his imperfect creation just as the drawing is Jamie’s.
Paterson puts "(thank god)" in parenthesis, reminding us of some higher power at work in the background. However, the fact that "god" is written without a capital 'G' implies that there may not be one omnipotent creator of the universe.
one inch from home/we couldn't get the air to him
The "shake" is the only remnant Jamie has of his difficult birth. The phrase "one inch from home" implies he was so close to being completely fine and safe when this happened, just as all is going well until the very last minute. The use of "we" also suggests the collective effort of his family and medical staff willing him to live.
he's all the earth/and sky for breathing-space
The next stanza brings us back to the present with Paterson shifting to the present tense. The use of "he's" is ambiguous and implies that he has a lot of space to breathe in, but also that he is all the world to the speaker.
Paterson uses repetition in "breathing-space" and "breath" to emphasise the sense of having space and time. He takes it further in the following line: "the whole damn troposphere" which extends the space into the layers of the atmosphere. And yet, despite all the space, all the air available, Jamie still struggled to breathe. He still has a "flutter in his signature" – a slight imperfection in his identity. The use of "damn" perhaps conveys the poet's frustration at this.