The Waiting Game

Elvis is waiting. He is waiting for me to call him to continue walking.
Good Boy!
But....
Several months ago, I was a little frustrated that he would charge on ahead of me. Not a problem, until we got to a point on the walk where the trees opened out to a clear area. That was where I normally see the deer and very very occasional Boar. His arrival before me scared everything worth photographing away, leaving me little or no chance for a snap.
So, I started telling him to stay at a point some 20 metres before the clearing, so that I could quietly and carefully go on ahead and try to snap the wildlife. After a few times, it worked.
Then Elvis decided that this was a new game - the Waiting Game. 
He stopped automatically at the correct spot each time, without being told to stay. Excellent Good Boy!  Then, a few days ago, he suddenly started doing it throughout the walk. I notice that he is no longer walking or sniffing the path, as he is about 50 metres behind me - waiting. I call him, he bounds up as fast as he can.  I praise him (long ago stopped giving treats, just the praise is enough). A few metres later, he does it again, and again, and again......throughout the whole walk. 
I have yet to decide whether he thinks it is what I want him to do, or whether he has created his own game where he gets to run much faster between stops, thereby getting more exercise. 
The result is that our strolls in the countryside are constantly punctuated by stop/start events. Not quite the Walk in the Woods I planned.
Now how do I get him to only ‘stay’ when I want him to, rather than when he just fancies playing?  Over to you........