The speaker directly addresses his father in the last stanza of the poem. However, generally, he is addressing all men (and I do mean men, as I do not think that women figured in Thomas's imagination for this poem) who see their lives coming to an end.
He considers different types of men, though he does not directly connect his father with any of them. First, there are "wise men":
Though wise men at their end know dark is right,
Because their words had forked no lightning they
Do not go gentle into that good night.
These men know, because they are wise, that they must die ("know dark is right"), but resist death because their wisdom has not had the desired impact on the living ("their words had forked no lightning").
Then, he contemplates "good men":
Good men, the last wave by, crying how bright
Their frail deeds might have danced in a green bay,
Rage, rage against the dying of the light.
The "good men" are described as both friendly ("wave by") and sorrowful because they are "crying." They cry because their ambitions for a better world never came to fruition; instead, they were "frail deeds." The use of the modal phrase "might have" makes it clear that they had potential to create an environment that would have been fertile for hope ("a green bay").
Next are the "wild men who caught and sang the sun in flight, / And learn, too late, they grieved on its way..." These, arguably, are the ones who loved life too much and lived very much in the moment ("sang the sun in flight").
Finally, there are the "grave men...who see with blinding sight / Blind eyes could blaze like meteors and be gay..." These are the men who were serious, perhaps even prone to sadness, and failed to grasp life's meaning, only to understand in death. The irony is caught by the phrase "see with blinding sight." Their vision, or understanding, becomes so exquisite that it [blazes] and they become "gay."
All of these men realize the finality of death and have epiphanies on their death beds which could have improved their lives; but, alas, it is too late. Thomas switches the final line of each stanza back and forth between "Rage, rage against the dying of the light" and "Do not go gentle into that good night." The message reads as a mantra. The latter, "do not go gentle," reads as an entreaty, while the imperative "rage, rage" is a battle cry, addressing the anger within each group of men -- all of whom may represent parts of the narrator's father:
And you, my father, there on the sad height,
Curse, bless, me now with your fierce tears, I pray.
Do not go gentle into that good night.
Rage, rage against the dying of the light.
His father is "there on the sad height," or the precipice from which he stares "into that good night." It is the same place from which the others see "dark" and develop "blinding sight." He has "fierce tears." What is interesting is that the father is described only through the actions he performs while on his death bed. There is no clue to his interior state, or what he may be thinking, as there is with the men that the narrator conceptualizes. This suggests a disconnection with the father that the narrator hopes to overcome with any form of communication ("curse, bless, me now with your fierce tears, I pray"). With "I pray," the narrator is given the final power and delivers -- in sadness, desperation, and false hope -- the full mantra: "Do not go gentle into that night / Rage, rage against the dying of the light."