I've done a bit of kids teaching.
Here's the Ski Instructor's mantra; Safety, Fun, and Learning. Keep the kid safe, make it fun, and learning will happen. (Kinda obvious I know, but it helps to reinforce the basics.)
I recommend using an Edgie Wedgie over some of the other tip retaining devices out there. They cost between $10 and $15.
These help hold the ski tips together so the kid can easily make a wedge. However, they allow independent movement of the skis so the feet and hips don't get "locked up." They can also pop off if anything happens. Some other types of devices don't allow the independent leg action and that can hinder movement and learning.
I don't recommend the use of tethers, especially the ones that go around the waist. Those pull the kid back into a backseat position. If you must use tethers, connect them to the tips of the kid's skis with them routed around the outside of his legs. This way you control the skis and not the skier. I've easily controlled 6'2" 280# disabled skiers this way. Again, I don't recommend it because you are doing the skiing for the kid, he isn't learning to ski. He's just going for the ride. (They are nice to have in your back pocket on the slopes just in case. Especially with a small kid.)
What I highly recommend you do is go out on the flats and play. Your front yard is great for this stage. (With snow of course.
) Start out in just boots. Run, skip, jump, throw snowballs, hop, side step, play hide and seek, just make it fun for the kid and yourself. Then do it all over again with one ski on. Throw in some scootering and sliding on one ski. Then switch feet and do it with the ski on the other foot.
Then put on both skis and do it some more. When he gets tired, take a break. You want him to learn on his schedule to make it fun. You can even get a rope and pull him around like a water skier so he gets use to sliding. (If you attach it to the tips of the skis, he'll learn to stand up while sliding. You can use some tiny C-clamps to tie the rope to.) Finally start sliding down very very gentle slopes. Just enough for him to get moving. There should be a run out so he'll coast to a stop. While sliding just have him look where he wants to go and see what happens. Run back up to the top of the "hill" and do it over and over and over and over.
That's about it for starting. Have fun, take it slow, and let learning just happen.