Make sure you shift your weight, if you swing righty, onto your left side as you contact the ball. The more you shift your weight onto your left side the more solidly you strike the ball.
nh43211 is correct. You must get your weight over to your left side. Also, try holding your wrist-cock longer on your downswing. Fat shots occur often when players try to use their hands to hit the ball. This is called "casting" or "flipping". Hold that wrist-cock as long as you can and use the turning of your body/hips to square the club face to the ball. Your wrists will un-hinge on their own without any conscience effort. This is a hard thing to fix, so be patient at the range.
It's not even sure he needs to shift back. Maybe he is sliding in the back swing. If you have a slide and get rid of it Im pretty sure you don't need to shift much weight to your front foot.
Many thoughts on this subject though, but I don't like the idea of shifting either way for my self. The natural turning of the body back and though puts more weight on front and back foot in forward/backswing in my opinion.
But there are many ways to swing the club so a video might be helpful.
Aside from the technical fixes that others have posted, use some mental pictures here, too.
When a player hits a crisp iron shot that is properly struck, the clubface hits the ball first and then continues through impact in a slightly downward motion into the turf. Picture a slight descending blow being placed on the ball, rather than thinking you have to scoop the ball up in the air. The loft of the club, the energy you impart on the ball, and the backspin the ball will have will get the ball up in the air and keep it up there.
To get you headed in that direction, play your mid and longer irons in the middle of your stance and your 8-iron, 9-iron and wedges about an inch behind that.
The simple answer is quit hitting the ground behind the ball, and start hitting it in front of the ball.
The question really cannot be answered because all we can do is quess which one of the hundred mistakes you are making. It could be as simple as your ball is to far forward in your stance and the swings bottoms out behind the ball.