A classification system called "Tanner Stages" has been developed to categorize how far along a boy has developed. The Tanner Stages don't address all the pubertal changes, but they do focus on genital development and pubic hair growth.
Stage 1 is for children who have not entered puberty, and no outward physical signs of puberty yet appear. In Stage 2, light, sparse pubic hair begins to grow, mostly at the base of the penis. Both the scrotum and testes enlarge, and the scrotal skin texture changes and can redden. During Stage 3, the pubic hair begins to coarsen and darken; it also spreads to the junction of the pubes. The penis enlarges, mostly in terms of length and a bit less in circumference. The testes and scrotum also continue to grow. By Stage 4, pubic hair distribution covers much the same area as on a grown man, but it likely isn't as thick. The penis has a major growth spurt, in both length and circumference. During this stage, the glans penis also develops. Testes and scrotum continue their growth, and the scrotal skin darkens. When the boy reaches the fifth and final stage, he is essentially out of puberty and is a fully developed adult. His pubic hair has fully come in and his genitalia have finished growing.
It is entirely natural for a boy's genital development and pubic hair growth to be in different Tanner stages at the same time. A boy will likely hit his first skeletal growth spurt during Stage 3, which usually occurs roughly two years after the onset of puberty (the first signs of Stage 2). This first growth spurt lasts two to three years and a boy will gain about 25 percent of his full adult height during this time. After Stage 3 of puberty has begun, a boy will also start developing muscle mass, hear his voice crack and may get acne. Once a boy reaches Stage 4, hair will continue to grow on other areas of the body. He will also have a second growth spurt, coupled with growing muscle mass that will give him his full adult frame.