I teach piano and I do not recommend instrument lessons until the child develops an interest in the instrument and is enthusiastic about learning. For piano, this means children who can sit and be engaged in a single activity for 30 minutes and children who can read. This usually falls around 5 or 6.
For children younger than 5 or 6, I recommend introducing them to different types of music, instruments, and rhythm and making music FUN. This means listening to different stations on the radio, dancing to the music, playing "drums" (pots and pans) fast and slow, and learning the different types of instruments and how they make sounds. Additionally, this includes singing - his own built-in instrument!
Lastly, in choosing an instrument for your child to start. Consider your tolerance level for listening to the child practicing. Beginners practicing drums and string instruments tend to be jarring to the ear.