Puppy Litter box training

Hello,
 
I am looking for suggestions.  We are litter box training our puppy.  He's 7 months old and has been doing really good.  He had a few weeks where he just started to forget where he was supposed to go, and he was just being stupid in general -- then we got him neutered.  All that changed, thank goodness.
 
The problem is, when he goes to poop in the box, he'll turn in circles, poop, and jump out of the box.  That sounds fine, but lately, he isn't finished or is in the process of finishing when he jumps out, so some of it ends up on the floor.  Any ideas on how to train him to stay until it's all out and in the box?
 
Thanks,
Keith