He has his head in the clouds.
More Traditional Proverbs:
Never put your trust in slogans.Traditional
The road to success is lined with many tempting parking spaces.
Traditional
For want of a nail the shoe was lost; for want of a shoe the horse was lost; for want of a horse the rider was lost.
Traditional
Keep your mouth shut and your eyes open.
Traditional
A small key opens big doors.
Traditional
Don't change color to match the walls. Look like you belong and the walls will change color to match you.
Traditional