During puberty a boy starts to produce sperm and will begin to ejaculate. If the erect penis is stimulated, muscles contract, pushing semen (containing sperm) down the urethra and out the tip of the penis. This is called ejaculation
Sometimes ejaculation can happen during sleep. This is called a “wet dream”. Wet dreams are the body getting used to its new sperm production and usually happen less often as your child gets older.