I really don't see why this keeps getting said, but Kody referring to Garrison as his din isn't something that started a few episodes ago. He has always referred to all of his kids as his kids, Garrison included, or the kids of their mother, Robyn’s kids included. We have several seasons of evidence of this.
The question is, why do so many want Kody referring to Garrison as his son or his boy to be a new development? How does ignoring what's been said on the show for several seasons in favor of fanon enhance the viewing experience?