Try having your son step into the batter's box & go ahead & take his step leaving his feet there, then relax & shift his weight backwards onto his back leg. When the ball is pitched & he is ready to swing, he simply transfers his weight to his front foot as he is swinging. Thus, eliminating stepping at all & simply using weight transfer at the swing. This will also eliminate stepping out too far or stepping out away from the plate. I use this technique with several of my student batters with stepping problems at the plate. The weight shift at the swing willl also put the power back into the swing...

As he is making his stride forward, make sure his hands and arms are moving back and slighly up at the same time. This is known as the stride and trigger. By moving the arms and hands back will help in keeping weight back.
Also make sure the front knee stays inside his foot. Another key apsect in keeping weight back

You should ask 10 year MLB Veteran Brian McRae. You can e-mail him at talk@ifungo.com and he will answer any hitting questions you have.

It depends how old your son is. If he is younger than 12, I wouldn't mess with his hitting mechanics. Hitting is such a mental thing, and teaching mechanics to young hitters just confuses them. Young hitters need a clear head, they are more sucessful when they are given practice time and their confidence boosted. A lighter bat might also help. The appropriate weight bat decreasing the occurance of bad hitting mechanics and poor timing.
Also one drill that my 9 year old LOVES is he hits wiffle balls or plastic training balls with his regular wood or metal bat. I throw soft toss from about 15 feet sitting on a 5 gallon bucket. Find a place with a fence and locate home plate 60 to 90 feet from home plate. Have a Home Run Derby, Call the Home Runs like your favorate Baseball anouncer.