I saw an earlier version of this post where you went a lot more into the lore you were trying to work through (and had a useful link to some earlier information from Fernehalwes). How come you deleted it?

In any case, as Vidu said, the Keeper family name is going to have priority as it is a family name, while Seekers don't have a continuous surname over generations - the tribe letter serves that function, and your character has rejected that, so you're not going to be using it as a couple.

So your boyfriend's character won't need to change her name, and it just comes down to what you (or your character) values about their name. You've already abandoned normal naming format, so it's down to whatever you/he would prefer. Did he choose to have his mother's-name-as-surname because he really wants to show that link, or just because he had to use something as a surname after abandoning the Tia title? Does he want to keep that name or change it to reflect his marriage?