someone who takes off easily

• It communicates a lifestyle of moving frequently, rather than a readiness to move, but someone who doesn't stay long in one place is a rolling stone (from the proverb "A rolling stone gathers no moss," which has a rather convoluted history).
• Not an idiom, but the German loanword wanderlust has been adopted into English: an insatiable urge to travel.
• There are some idioms that communicate the opposite, which you could use in negative. The kind of person you describe is not tied down, or is not a homebody or a stick-in-the-mud.

Perhaps something combining two expressions:

He's always ready to pack up and go/leave/move at a moment's notice.

or

He's always ready to pack up and go/leave/move on the spur of the moment.

Your "itchy feet" is similar to

wanderlust

A strong desire to travel m-w