How can you safely make your penis grow?

A boy's penis develops/grow's with him, but quicker during Puberty due to increased hormones. It continues to grow right through puberty and beyond, but stops at around 20-years-old. After this the penis grow's no more.

Because the penis does not have growth muscles or bone it cannot grow any larger once normal body growth has finished.

A penis and testicles can shrink and enlarge due to body temperature. This is not a permanent fixture but perfectly normal and healthy. During cold weather the penis shrinks towards the torso and during hot weather it expands away. This is it's way of regulating the best temperature.