It seems that some dogs, instead of learning "I have to go outside ALL the time", instead learn "I can't go to the bathroom inside this particular house". So they will know not to go inside your home, but won't really translate that to not going inside any building. Does that make sense?
Before you take him into diffrent buildings, do you walk him around outside so he has a chance to go outside before he goes in? If he goes outside, praise him like crazy. If he goes inside, calmly take him outside and wait for him to finish outside, and praise him when he does. If I catch my dog going inside, I clap my hands and tell him "no", and take him outside. That's the extent of my discipline, I never rub his nose in it or anything. If he goes outside though, he gets a treat and lots of praise.
It honestly sounds to me like you just need more time and to work with him more untill he gets it. If you can, try taking him to friends houses and into diffrent buildings and working on his training there as well, so that hopefully soon he will understand that he always has to go outside no matter what building he's in.
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