I would advise teaching the correct grip as this can affect the whole swing. Also the correct ball postition and body alignment are advisable too; try to get enough things right before you work on the swing.

The younger they are the more flexible they seem to be so you can teach them the correct turning mannerism more efficiently. I would not let him take the club back more than hip height at first. The important part is letting him hit the ball forward. If he tries a full swing he will have a miserable time hitting it sideways and he could lose interest in the game. Besides, a fuller swing happens naturally through exercising the body and the enthusiasm that a good basic start provides.

Introducing the correct hand and wrist action is important too otherwise the elbows tend to bend

Get him into the habit of holding the finish position until the ball has landed as this will also help his balance as well as setting an example of good technique.

Finally, a cheeky tip If you have lessons yourself, take him along with you when you see the pro. Give him his club and let him hit a few balls around to keep him interested. The pro will probably takes an interest and correct your lad occasionally at no extra cost. Well, it's worth a try ...