We need a sticky posted at the top of this forum.
Most common problems:
1) You should be moving towards home plate as soon as your leg lifts up. This is not necesary, but is highly recommended to increase forward momentum. (Look at 2:40)
2) Opening up too early. Look at 2:50, he is completely opened up before he lands.
3) Too short of a stride (your son's stride looks like a good length though, just make sure it stays close to his height).
4) Hip and Arm Rotation. He should land on his front foot, square his hips with the plate then rotate his upper body. This is where a lot of speed is created. He rotates both at the same time, look at 0:33.