He wants to fly before he has wings.
More Dutch Proverbs:
Patience surpasses learning.Dutch
Take a horse by his bridle and a man by his word.
Dutch
He that comes unbidden goes unthanked.
Dutch
It is good to go afoot when one is tired of riding.
Dutch
It is hard for an empty bag to stand upright.
Dutch
Give him your finger and he will seize your hand.
Dutch