Puppy Will Pee Outside But Not Poop?

I have a pet Schnoodle who is 12 weeks old. I have had him since he was 8 weeks old. I am having problems with toilet training him to do solids outside. He has been fine with liquid waste and will go whenever he is put outside and instructed to do so. What could be causing him to constantly want to do solids inside and not outside? Any suggestions please would be great.

By Mark Linaker