Possible Clues:

• Beethoven's 'Kreutzer ___'
• A musical form
• Young man joins a group of reservists - it's all of a piece
• Music to play (not sing)
• Instrumental composition of three or more movements
• To your boy, an expression of thanks for the composition
• The next generation at a composition
• Sat on a kind of composition
• An expression of gratitude from your boy for the music
• Beethoven's 'Pathétique' is one