Except that is his own decision.
> Why bother?
Because a kid is a separate person, not an extension of your personality.
The relationship between a parent and child is not a democratic one. As a parent you have a responsibility to make choices for your child until they can make them for themselves. I do not agree with letting kids decide which developmental activities they will or will not partake in or how they will do so. Is that clear enough for you, sunshine?
It's not like there is an objective way to choose a "better" instrument anyway since half of the fun is playing with others who use different instruments.
Why would it affect their development if they picked say a trumpet, guitar or flute instead? They can always pick up a different instrument later.
It would be like if a kid wanted to learn python and you said "no! you must learn Java instead".
EDIT: Setting aside the above argument for a second, there are practical considerations too that make the choice of musical instrument a non-arbitrary decision. For example: the cost of the instrument, its portability (important if the band decides to tour) and of course the availability of instruction.