Taking a false oath when dealing with a psychological patient

Our elder brother is a physiological patient and on drug abuse. We tried to convince him for treatment but he refused. We started an outdoor treatment with a physiologist. In this treatment we teamed up with the psychologist and started his physiological treatment in which we perform different types of therapies. We changed our behaviour towards him and made different approaches. He is almost 75% recovered now.

Recently he started doubting us that we are doing some kind of treatment without him knowing. He started fighting with us and we denied doing any treatment as the doctor told us not to tell him at this stage. He is now asking us to put our hand on the Quran and take an oath that we are not doing any treatment for him. The doctor is advising us that after a few months, when he is fully recovered, then only should we will tell him the truth.

What does Islam say? Should we take a false oath to satisfy him for sake of his recovery and later, when he is fine then tell him the truth?