Actually the Fauvres give him as an example of a Social Four. I don't think their argument is the best, and that's not why I think that could be his type, but they are people who type him that way.
If he is indeed a 5w4, I agree on the sx/sp. And this is where enneagram types plus instincts get tricky. Because most- I want to say all but I want to allow for exceptions- most fours want intimate soul mates. It's like the lifetime Four fantasy- to find that special someone. It's why most (not all) fours identify with the sexual instinct descriptions when it's described as 'one to one' and 'intimacy'.
I'm going to give an example: I have a friend who is a social four, and probably a so/sp at that (it's how he types himself), and he has been going on and on for the last five months about his former best friend/girlfriend who got away. He is obsessed with this girl and they were best friends. They spent every waking moment together. They were very intimate and attached at the hip.
So why isn't he a sx dom? Because it's different for sexual fours. As I mentioned before, they are extremely idealistic and selective when it comes to romantic relationships. They are looking for the perfect diamond. So what you will see with sexual fours is them getting really excited about someone, and then just as quickly discarding them for not fitting their ideal. For this reason sexual fours often don't go past the starry eyed phase in a relationship. It's not that they don't want a relationship and don't want to commit- they do, very much- but they will only settle for the perfect person.
Kurt did not do this. He got married and had a baby young and had a best friend he spent all his time with. I have yet to see a sexual four behave this way. Others will likely disagree, but I'm just going by what I've noticed. Something I'm beginning to think, actually, is that sexual second fours probably often pair with sexual doms of other types, because they have more in common with them than they do of sexual seconds of other types. I know, for example, that I really don't relate to sp/sx of other enneagram types, moreso sx/sp. And it's part of why I'm starting to think my husband could be sx/so rather than so/sx. (That and something from the Bea Chestnut sexual seven description.)
(I'd like @Noll's and @cascadeco's take on this.)