housetraining question

We adopted a three month old miniture poodle mix this week. We are trying to house train him. When we bring him out to the designated spot on his leash, he will not go potty. This is a spot that he chose, and used once. In the last few days he has "held it" for up to seven or eight hours. When we take him out twice a day for a walk, he will go potty at the park on our route. He has only had two indoor accidents, but we are concerned that he won't go when we take him out. Is this unhealthy, and what can we do to get him to go in his spot in our yard? Thanks,
   
Renee Fortino