Your son's mechanics are generally OK, but three things stick out to my eye which should probably be fixed.
His lift leg could come up higher in the "balance" position. It's below the hips, which is a touch too low. Aim for hip-level or higher. (Think about Nolan Ryan's leg lift and then look at your son's.) Ryan's is way up high. Your son's is very low. You don't have to be as high as Ryan, but it should be higher, in my opinion.)
He strides open, which prematurely opens the hips -- but he lands toes pointed to the target, which is excellent. Place a piece of tape down the center of the mound out about 5 feet out and have your son stride out, ball of foot landing on that strip of tape. This will help "close" him up a bit.
He lands on a soft front (stride) leg, and doesn't brace up properly, which is why it looks as though he's "walking" off the mound after he makes a pitch. Your son should stride out and land with a bent knee, BUT ... as he delivers the pitch, the knee should gradually firm up or become straighter as the body and arm come over the top of it and decelerate in the follow through.
I'd just work on one or two things at once. It'll help him retain it and make it stick faster.