Potty training a dog in a fenced back yard

Question
I just got a Rat Terrier who is 8 weeks old.  He already can go through the night without going potty.  When I put him outside in specific part of the yard he goes pee pee just fine.  Right now I am carrying him to the spot.  After he goes I play with him in the yard or my kids do.  This usually is for an hour or more at a time.  During this time he might go poop or pee a time or two.  This happens so quick that it's hard to catch him in the act and put him in his potty spot in the yard.  QUESTION:  How do I train him to go to this spot in the yard while he is out playing.  Do I limit his play time before putting him back in his crate to reinforce the potty training.  Your help is appreciated, thanks.

Answer
Hi Darin,
 You basically answered your own question.  You are doing an awesome job.  Yes limit your play with him.  He is only 8 weeks old and you won't have to do that for very long.  He is like a sponge right now and just keep putting him back in his spot.  You will learn his posture for pee and you will learn his look for poo.  Just tell him NO and put him in his spot in the yard when you see that.  You won't have to do this for very long it seems.  Smart little dog.  
 Keep up the good work.
Tammy