What a curious story, Somsubhra! Perhaps, he is in his own world that makes no sense to an outsider. Who knows?
Your story reminds me of a man who used to run through our city. He appeared to be talking to himself as he ran. Occasionally, he would call out to passing cars, waving hello enthusiastically. Many thought he was mentally ill. A reporter finally tracked him down and asked about his unusual behavior. He said what looked like self-talking was really his prayers for the people passing him in cars. He waved to those he thought looked like they would most benefit by a friendly gesture. He said his unusual running routine was his prayer time, and he chose to pray for strangers.
Maybe, your guy talking to his fist is different. Or, maybe, he is sending prayers to those he can no longer call.