Child runs fever every time returns from noncustodial parent home

Patient: My stepson runs fever every time he comes back from his mother’s house. He is 4 years old and just started living with us a couple months ago after his mother decided to give us custody. After he is back from her house for a couple days the fever is gone and he is fine, but every time he comes back from her house he runs fever for the first couple days. And I do mean every single time. I know she smokes cigarettes, and no one here smokes. I asked her the other day is he ever sickly or running fever often and suggested he may be allergic to cigarette smoke, but she assured me he had no allergies and claims he is running fever because molars are cutting through. I have checked his mouth and do not feel any teeth near the surface. That combined with the fact that he never runs fever here except when he has just come back from his mother’s house concerns me. I don’t feel like it is my place to tell her that he only runs fever when he comes back from her house because until we officially go to court to get the custody changed she could still change her mind and take him back. Because I am concerned something is causing this I do not want that to happen so we have to just keep quiet until after court. What could be the cause of this?

Symptoms: Fever