He needs to gain more momentum by drifting when he is lifting his leg up.
This will increase his very short stride and get him more momentum at the hitter and throw harder maybe.
Also Step back not to the side this will help and get everything at the target. and make it easy to drift to the plate when lifting the leg.
Also your son is throwing with all all and upper body no lower body. If you get more momentum by drifting towards the plate when lifting the leg then you will see your son throwing harder then ever before.
Look at the Randy Johnson video at the bottom of the page and how he is drifting while lifting the leg up. Work on it. It takes alot of practice too.