Ok, change it up then. Keep the ball as a treat and throw another toy for him. Then make trades off of the ball...

I understand you want to try and find new ways to exercise him, thats great, really it is. BUT.... Fetch is something that you are going to have to teach him later on down the road, and you dont want him to develop bad habits such as hard mouth, or something like that before you have taught him the pre-requisites... have you introduced him to water? Walking a dog is great exercise, letting them run around freely and burn some energy is good for their attitudes. Get him in some water, let him swim around you while you stand waist deep. Let him be a puppy. Retrieveing will have its time and place believe me! Right now you want to keep it fun and intresting.

About the sit/ stay, he remembers, he just doesnt know that he has to do it everytime, he doesnt know thathe "works" for you, the boss. Right now he is going to do it for a treat, and not because he is trained to do it... thats where the repetition comes into play, you have to show him that your the "boss".