Use a weight on the fishing pole and let him cast it on land. He'll learn to cast this way. Put a round ring, or a bucket, or even a towel on the ground about 20 - 30 feet in front of him and have him cast to see if he can hit it. This by itself will keep him busy for for hours.
Then, when he really get's the casting down, put the hook on and do it for real.
I still enjoy casting on dry land. That's how I taught my kids when they were old enough to just start walking.
Edit: You can put several markers on the ground at different distances. Assign points to each marker (like a gigantic dart board). Lowest points are up close, higher points are farther out. Then he casts one, then you cast one. Keep score. He'll have a blast ... and who knows? ... he may even beat the socks off YOU!