Does dua really work?

My brother who's been making dua for 10 years, day and night, for a cure and nothing has changed so he said to me he will never make dua again as it's a waste of time. He said "I wasted all that time and tears for nothing...." My question is: Can anyone prove to me dua really works? Because God said call me and I will answer. Islam is confusing and don't tell me there's three ways of answering dua.