Prosecutors say Trevor Gocan, 57, punched and kicked pensioner James O'Neill, 74, before leaving him bleeding on the floor at an estate where they both lived in Covent Garden.
The boy was celebrating his 12th birthday and had come out of a Zara store when he froze on the spot as he saw blood dripping out of 74-year-old James O'Neill's mouth and head.
Trevor Gocan, 57, is accused of punching and kicking pensioner James O'Neill, 74, and leaving him for dead following a dispute over a gate at Odhams Walk estate in central London.