He could become immune if he sleeps with her more :3 But that is up to you I guess if he wants to do that ^_^ you control him. She has to love someone. Also the Asylum needs tending. Himori has two people, she could go for, the Siren or Griff, but griff lives with her so she has a higher chance of falling for him :3
Maybe he can enjoy her holding him? She will always provide him the Antidote for her pheromones.... he may not like to cuddle but he enjoys Himori's embrace, and she may need it more than he does, as she was never allowed to be close to someone due to her pheromones.