Exercise and Feeding

Question
I have a six month old German Shepard/Husky mix.
He's very active, very playful but he's kept on a leash most of the day. Every day I bring him to the park with my mixbreed husky of 10 years and they play. I've read about husky and german shepard exercise. But it seems my puppy does not get enough exercise, that or he just never tires. He can run for hours.

Is this normal? he's always excited and almost never sits down. When I bring him to the park he just runs and runs and chases the husky.

Also, how much should I be feeding him? We feed him 4 cups of puppy chow a day, is this enough? Because it seems that he's not growing any larger. Could this be because he is a mixed breed? Because both german shepards and huskies are large dogs... At the moment, he's as tall as my husky, and wider. Seeing as he's already this big, do you think he'll be growing any larger? And will mroe exercise be needed?

thanks

Answer
Some dogs are just more energetic than others.  they are only a puppy once.  Let him run and enjoy himself.  

I can't say if 4 cups is enough food for him, and the amounts on the package are just a starting point.  You judge how much to feed by the dog's ribs, see http://www.puppychow.com/products/popup_body_condition.aspx  Often shepherds refuse to eat enough to keep themselves any heavier than the low end of normal. Don't worry about it.  A little lean is much better than overweight.  

When you run low in puppy chow, start mixing in the adult chow you are feeding the other dog.  An early switch to adult chow allows more time to develop sturdy joints.  He will still achieve his genetically determined size, just not quite as soon.