I am a single father with a teen son. I have started a relationship with someone who is nude most of the time at home. I have started being nude at her home with her. How do I explain this to my son? My GF has a son his age as well and is comfortable with this. How would I explain this to my son or get him comfortable with the idea of this as I am thinking of being nude at home myself.

Naturalists (they generally prefer that to nudists) have a preference for not swaddling themselves in cloth. This Bing search has several articles that give good reasons for practicing nudism:
nudism reasons - Bing

I suggest reviewing them and use them to explain to your son (if he is a teen he's old enough to understand) what its all about. He's old enough to also understand societal mores about public nudity and the difference between private and public.

First you can explain it to him, but you can not force him to accept it and of course can not force him to be OK with it, and want to "experience it". Also guidelines need to be set up about answering the door, behavior when others are visiting and more.

Be sure everyone is on the same page, I will be honest Children Protective services will not be, for example, you being nude in front of her son, or her being nude in front of your son, depending on his age. So if you have a ex that wants to cause trouble, CPS is not really that understanding.