Min Pin pooping in the house

Question
My family has taken in a male Min Pin that was a stray. Very quickly we realized it was already house broken and we suspect crate trained as well. We also have a Catahula that lives in the backyard. The problem we are having is the Min Pin does not want to poop outside in the backyard. We can take him out, he will pee, bring him in and then about 5-10 he poops in the house (when we are watching). If we take him for a walk to the park he has no problem pooping while there but for some reason will not go in the backyard. We are at our wits end on how to resolve this problem. If you have any suggestions we are very open! Thank you so much for your time and help.

Answer
You may just need to try waiting him out once or twice.  Take him out right after he's eaten; that's when the bowels generally move and that's when the dog is most likely to have to go.

Have your mind set ahead of time that you're not going to be in a hurry, you're just going to sit there and wait (read a book, water the grass, etc) until he goes.  Then when he does, really praise, praise, praise while he's in the act of going.

You may need to repeat this several times until he gets the idea.