The last two notes
Listen to the last two notes of the "American Symphony"--a high, triumphant note, in octaves, featuring strings and brass (and probably the rest of the orchestra), followed by a low, sustained major chord which dies slowly (a "diminuendo", to you).
Now get to your CD or "record player" and put on "A Day in the Life" by the Beatles. Listen to the last two notes of that song--a high, triumphant note, featuring strings and brass, then a low, sustained major chord which dies very slowly. (I don't mean the spliced, looped bit at the end, just the song itself).
Now consider the similarities and the fact that Holland was supposed to be a Beatles fan in the movie. The endings are so similar that it warrants at least some consideration. Either:
1) Michael Kamen (the composer) deliberately put a Beatles motif into the American Symphony to suggest that Holland, the character, had been influenced by the Beatles in his composition (which would be perfectly plausible);
2) Kamen himself unwittingly and/or subconsciously took a "bit" from the Beatles. This is quite plausible, too, and is perfectly forgivable.
Kamen died well before his time in 2003, so we can't ask him.
Just another of those minor things that keep a person up at night.
(Editing in: I put the last 5 seconds of MHO and the last 5 seconds of "A Day in the Life", end-to-end, using Garageband, with a 3 second gap between the songs, played it to my class of 16-year-olds (who had just seen MHO) and they were amazed at the similarities in the endings.)