There's a wide range of normal for this milestone; some 2-year-olds may do it easily while some 5-year-olds still struggle with it, so it really depends on the child. Help your little one learn by explaining that in addition to blowing a burst of air out of his mouth; he can also blow through his nose. To demonstrate hold a tissue under your child's nose and tell him to watch how it moves as he breathes. Then blow your own nose while he watches. Once your child seems to understand, hand him a tissue and tell him to take a deep breath while keeping his lips sealed. Then pretend he's gently blowing his birthday candles out with his nose. Like any other skill that takes coordination, practice makes perfect, so let your child try blowing his own nose first when the need arises and step in afterward if necessary.