Ceiling wall damaged due to water leakage from above flat

Dear Sir, We stay in coperative housing society. We have been facing problem of water leakage in our flat (one of the bedroom) from the flat above us since last 8-10 years. Even after notifying several times he hadn't taken the matter seriously and root cause of leakage was not resolved, although we repaired ceiling of bedroom many times in last few years on our own expenses. Now the condition of ceiling wall has gone bad to worse and we can't use our bedroom anymore (as dust comes out frequently due to leakage). Also, the area of leakage is spreading drastically. We have notified our problem multiple times through letters to the house owner and managing committee of the society to address this issue. However, this issue is completely ignored by them. Since the flat owner is the secretary of the society, taking undue advantage of his position and delaying the process to find out root cause of the leakage. Also the flat owner is doctor and from last few months taking the shelter of covid-pandemic he is deferring the issue. He is not allowing anyone (including plumber or professional expert) to inspect his flat and gives explanation that he had not made any alterations in his flat. We are completely harassed by his non-cooperative behaviour in handling this issue. Please suggest what should we do to resolve this matter ASAP.