As they say, no one knows a child better than their mother. Since Tom had always lived with his mother, she should have been one of the people who knew him best.
Since Tom's mother stated he was cheerful and unlikely to have suicidal tendencies, the possibility of Tom's suicide significantly decreased.
Assuming Tom did not commit suicide, that would prove he was unaware of the poison in the bottle of mineral water.
It's very likely he wasn't the actual Poisoner, considering the forty US Dollars found in his pocket.
Luke boldly speculated that the real Poisoner, to avoid being caught by the subway station's surveillance, hired Tom to place the box of mineral water at the station, and that forty US Dollars was the payment he received for it.
After placing the box of mineral water, Tom took a bottle home to drink and died from poisoning.
