Op, without knowing your family, your lives, how you treat DS, what your home life is like, how you all treat each other, how tidy your home is etc etc etc, it’s impossible for us to give “our thoughts”
Sure, on the surface, with your one sided post, it sounds very sad for you that he chooses to be this way.
But he’s also almost an adult, and we don’t know his side of the story. There could be a million and one genuine, valid reasons why you’ve never met her and why he never chooses to be at home.
My DS and his GF for example, choose to always be at ours. Because they’re free to do as they please here, they’re both almost 19, they have privacy, we treat them like adults. Whereas at his GFs, there’s a million and one rules imposed, everything is regimented, they’re not made to feel particularly welcome. They’re a perfectly nice family, but my DS and his GF just don’t feel particularly comfortable there, they’re always on edge in case they get told off for making a mess, or using the wrong crockery 🤣 and they don’t get much privacy.