Hi,
Normally puberty starts in boys at 12 years of age and in some kids it would start earlier. The early start of puberty is called precocious puberty. But as per definition of precocious puberty, nine years is considered the cut off age and therefore your child may not have this condition. In any case, it is better to consult a pediatric endocrinologist for a detailed check up. For more details on precocious puberty, you could visit this link :
http://www.mayoclinic.com/health/precocious-puberty/DS00883/DSECTION=causes
Solace
He could be starting puberty at 9 years old. You might want to take him in to see the pediatrician for a check up. Can you think of any other reason he may be acting out?