Hi,
Boys normally develop at different times. Some may start developing as early as 9. Others may not start developing until 15 or even later. The age varies from person to person and depends on when he enters puberty and his hormones start doing their thing. Growth in penis size is just one part of puberty, which also includes such changes as pubic hair development, testicular growth, muscle development, and a growth spurt.
Take care and regards.
It is stupid to believe size matters. Be grateful he is healthy.