T. has a one-track mind. He is a school psychologist who thinks that the solution to all teenage special learners' problems is for the teen to attend all his planning meetings with school personnel. He is so stuck on this idea that he can't even recognize when the parents are already allowing and encouraging the teen to take an active role in the family's advocacy efforts.
What can I call the thing the one-track-minded person is stuck on? It's an idée fixe but I often have to communicate with people who are unlikely to understand idée fixe. I end up calling it T.'s thing. Strictly speaking it is an obsession but this give offense.
T.'s _________ is that teens should attend their own meetings and advocate for themselves.
I don't mind recasting if it helps me communicate with an unsophisticated audience.
Edit: I think Lee Leon cracked it:
T. feels it's his mission to get teens to attend their own meetings and advocate for themselves.