From the Hollywood Reporter article I linked:
Hollywood Reporter wrote:[Jacob] Buster will play Peter, who suffers the same affliction as Eleanor. When his father tests a potential cure on him that backfires, a terrifying monster is unleashed.
That's all we know. It sounds like it either causes him to physically transform or causes him to go insane/feral. Maybe it simply kills him and turns him into an undead, as a counterpart to the novel's undead Håkan.
Personally, I don't really care about them changing the details of how the infection works. Obviously some elements have to stay (the invitation rule has to be kept in order for the title to make sense), but the most important thing to me is that it's still presented as a gross and life-ruining disease and not as some romanticized superpower. If they stay true to that spirit of the original story, I think it's fine if they tinker with the smaller details in order to fit the story they're telling. After all, TA and JAL already did that to an extent in the film (having Eli not sleep in blood, for example). Heck, JAL himself has already introduced a pretty different version of the infection: in
What Kept You So Long?, the disease works very different from how it does in LTROI simply because it needed to fit the story he wanted to tell, and that's in a piece of writing that's supposed to (at least loosely) take place in the same continuity as LTROI and LTODD.
