Update on this: My son had a comfortable journey to London and reached safely.
I did not receive any call from AI three days before the travel. My wife visited the AI office in BLR and they told that I would get a call on the day of travel (which also never came, eventually).
On the day of travel, my wife dropped my son in the AI ticket counter in the airport. She was not allowed in. The AI staff kept possession of the child's passport and other documents all through the flight. He was made to sit in the AI counter till about an hour before the flight. During the flight he was made to sit in a row full of kids. According to my son, the air hostess was quite kind and even prepared garlic bread and served it to him at his request.
In LHR, I was waiting at arrivals. An AI official accompanied him and after verifying my identity, got my signatures and handed over my son.
Overall my son had a very comfortable, safe journey and was well cared for throughout. Kudos to AI. Wouldn't hesitate to use this service again.