Puberty in boys

A good source of information below:

Boys and girls begin and end puberty at all different ages. What is considered 'normal' or typical varies a great deal. To get a better idea of how much variation is considered normal, it is important to understand how puberty is assessed.


Puberty, the natural progression from a child-like physique to an adult body, begins with hormonal signaling between the gonads (the testes in the case of boys) and a part of the brain called the hypothalamus and the pituitary gland. In boys, puberty usually begins between 9 and 14 years of age in the United States. (This is an important point to make as ethnicity apparently influences the average age at which puberty begins both for boys and girls.) If there is evidence of testicular enlargement before the 9th birthday or no evidence of pubertal changes by the 14th birthday, then those are reasons for concern and must be evaluated.
The first physical change that is noted in a boy is the enlargement of the testes and the lengthening of the penis. In the second stage of pubertal development, the skin over the scrotum begins to thin and redden and there is the beginning of a sparse amount of pubic hair at the base of the penis. Because most parents are no longer seeing their children fully undressed in early adolescence, these very early changes in boys may not be noticeable to the parent.


The third stage of puberty begins when there is increased amount of pubic hair that is noticeably coarser and darker than before. It is at this point that the peak height velocity or 'growth spurt' usually begins. This is the most obvious physical change and is typically about two years after the onset of the very first signs of puberty. The 'growth spurt' lasts about two to three years. A boy achieves about 25% of his final adult height during the growth spurt, and he will gain an average of 3.5 inches a year during this time. This compares to 2.3 inches per year in the prepubertal male. Another common occurrence in the third stage of puberty is the development of breast tissue. This is called gynecomastia, and it can be on one or both sides. For most boys this is a passing change that will spontaneously improve if the breast tissue is less than 1.5 inches across.


After the onset of the third stage of puberty, boys also gain in muscle mass, the voice deepens, acne frequently becomes bothersome. The fourth and fifth stages of pubertal development are marked by increased in pubic, axillary, and body hair as well as further enlargement of the penis and testes. For boys there is also a 'strength' spurt related to the increased muscle mass. Final height is usually achieved during the final stages of puberty. A look at the growth charts for boys ages two to eighteen can give you some sense of where your son fits in with his peers with regard to height and weight.

Source:
http://www.keepkidshealthy.com/development/puberty_boys.html