He wanted to resist, to tell her she couldn't promise him that. no one could promise that. There were ghosts in that house. There was evil, alive and breathing, and calling out to them both. Calling him to take his own life, just as it had called to him before. Just as it had called to, and claimed, Nellie. And their mother.
"Okay." He wasn't okay. Not at all. He was shaking. Barely holding it together. But he was with Theo, and he trusted Theo. If Theo went, he would go too. Because he wasn't going to let her go in alone.