This is a GOOD problem to have, because like Jane said, your pup WANTS to be interacting with you, but you are being boring.
I wouldn't sweat this. If this was my pup, I'd be happy that he was this playful, energetic, and wanting to push me to get his reward.
Jane gave some good pointers - also for a young dog like this, the rewards should come fast and furious. When expected to do long stretches of behaviours without reward, they lose their focus, or they think that they are doing the wrong things because the rewards aren't coming often enough, and GSDs being so smart, try to figure out what you REALLY want and start making things up and throwing all sorts of behaviours at your to hoping that they will hit the right one.
When teaching something new, make sure to really break down the new exercise into really small pieces and reward each one as they get it. For example to get them to heel backwards, you lure them back and your reward for following your hand with their head - reward. Then they shift their weight back to be able to follow your hand - reward. Then they move a rear foot back so as not to fall backwards when following your hand - reward. Then they make a bit of a shuffle move backwards - reward, etc - as you can see, if you are asking too much too soon before they understand, they can get frustrated and the frustration can express itself in some of the behaviours you see. But still good stuff!! What would worry me is a pup that shut down and disengaged because he was confused or bored. Yours is WANTING to work for you, he just not sure what you want.