How does Simon Armitage portray the relationship between father and son in his poem 'Harmonium'?

The poem starts off with a tone of reminiscence, with the harmonium 'gathering dust', showing that it has clearly been forgotten in the church for many years, echoing the type of nostalgic relationship between father and child. The speaker remembers how they used to sing together: 'father and son,/ each in their time, had opened their throats/ and gilded finches - like high notes - had streamed out.' However, it is clear that in the present time, they do not sing together any more as they carry the harmonium out of the church and the father makes a dark joke about his future death which the son does not know how to respond to: 'And I, being me, then mouth in reply/some shallow or sorry phrase or word/too starved of breath to make itself heard.' The mention of breath relates back to the singing, and the fact that he is now 'starved' of it shows a growing distance from his father and a growing fear of the inevitability of death, foreshadowed through the fact that the father is a heavy smoker, seen through 'his own blue cloud of tobacco smog.' Overall, the poem is an extended metaphor, comparing the dustiness of the harmonium tot he deterioration of their relationship, and, in turn, the deterioration of the father's life.