My son was a cocky jerk most of the time (with glimpses of the loving little boy he'd been) long before 18. At 22 after a year in the Air Force, he was back to being nice again....and now he's a dad. It's wonderful to sit back and watch, and know that in spite of it all----I did something right with him! :)
Don't worry, this too shall pass....yours will grow up just fine. Be patient!