What does "have a year for every trombone that led the big parade" mean?

In REVIVAL written by Stephen King, when its main character (who works at a recording studio) asks for a couple-of-months-break, his boss says with a sigh, "Come this fall, I'm going to have a year for every trombone that led the big parade." I have no idea what it means.

It means he's going to be 76 in the autumn ("fall"). It's a reference to a song from the 1957 music The Music Man by Meredith Willson (1902–1984):

Seventy-six trombones led the big parade
With a hundred and ten cornets close at hand.
They were followed by rows and rows of the finest virtuo-
Sos, the cream of ev'ry famous band.
Seventy-six trombones caught the morning sun
With a hundred and ten cornets right behind
There were more than a thousand reeds
Springing up like weeds
There were horns of ev'ry shape and kind.
There were copper bottom tympani in horse platoons
Thundering, thundering all along the way.
Double bell euphoniums and big bassoons,
Each bassoon having its big, fat say!
There were fifty mounted cannon in the battery
Thundering, thundering louder than before
Clarinets of ev'ry size
And trumpeters who'd improvise
A full octave higher than the score!

Lyrics via stlyrics.com

The first line at least has become quite famous, probably because of the catchy rhythm of triplets in the anacrucis (demonstrated in a YouTube video chosen at random).