He'll grow to however large his genetics dictate he should be. No amount of exercise or food will help him grow bigger. He needs just enough food to nurture him at this point and provide the calories for growth...over feeding will just cause a fat dog with a lot of stress on the joints.
The walks you're doing are fine. A dog can walk on pavement. Just don't run him on pavement. It hasn't really been proven that doing certain exercises increases risk of joint problems, but running on pavement is bad for all living things as it has absolutely no give for your joints.
You have to look at the food you're using and follow the chart it has on the back. If he seems to be getting fatter rather than growing, feed less, if he's super skinny, feed more. Pretty simple.