He says, "Son, can you play me a memory
"I'm not really sure how it goes
"But it's sad, and it's sweet, and I knew it complete
"When I wore a younger man's clothes"
The "complete" in the third line is a flat adverb, devoid of its usual -ly ending. Part of this might be to avoid an overload of syllables in that line. More likely, it's so that there's a perfect internal rhyme between "sweet" and "complete" (as there is between "joke" and "smoke" in a later verse).
Aside from technical considerations, as a flat adverb, "complete" indicates the degradation of the old man's memory. He himself says that he's "not really sure how it goes." In the same way that his memory isn't intact, neither is the word completely.
