The 38-year-old and Pitt, who have six kids together, celebrated Maddox's 12th birthday last week, reported Showbizspy.
"Maddox wants to be a guitarist and has got all the poses. He's been having guitar lessons for a few months and has a real passion for it. So Brad and Angelina figured a new guitar would be the perfect gift.
"Brad's very proud. He always wanted to be a rock star and now he's getting a chance to live out his dreams through Maddox," a source said.