Keeping the lats engaged, the back tight, and pushing hard through the floor should help him stop bumping the bar forward and causing him to fall forward. Explosive knee AND hip extension is desired; if he's only getting explosive hip extension and not using his legs to build bar speed, then he's going to send the bar forward. In my opinion, he should spend time doing pulls focusing on that leg and hip drive.
I hope that helps,
Javier A. Sanjuan
Olympus Barbell Club
Dear God, please help me lift heavy and be awesome. Thanks. Amen.