Start with showing him the grip, stance, and keeping the bat back (in proper position), then let him swing away. Like others said, make it fun for him, and as he starts showing more interest in learning, then start showing him correct swing mechanics. I think most importantly at this age is to help them develop good hand-eye corrdination...by playing catch with them, and pitching and soft tossing to them alot. Kids need a lot of one on one time at home to help develop their skills. They throw a lot at baseball practice, but they don't get enough hitting time.
I can't get out much with my 8 and 10 yr old grandsons and practice with them, and my daughter (who is divorced) doesn't have much time, so I got them the Ulimate Hitting Machine. (Soft toss machine sold by Beebe Sports)
They can set it up and use it by themselves and they both love it. Using it almost everyday has definitely made a difference in their hitting.
They don't use it as much now that baseball has started, and they said their coach is looking into getting one to use in his practices.
This is a good machine for young kids to use to practice hitting moving balls.
If you have a place to set a net up in your yard, I would reccomend getting one of these machines.
The best price for this machine is found here.
www.shop.beebesports.com