WOW, I think it is best that you just follow his lead right now. The key point in your story is that he took a break from his friends and he ASKED you to remind him to put on the tefillin.
Even though he stopped already and went back to the friends, who you said got into the Dorm Yeshiva (?), he did relate to you, communicate with you, trust you, etc. That is a step in the right direction. Baby steps are good no matter how long they last, they are good!