How odd. Obviously he can go on all fours but chose to walk upright.
A behavioural mutation? Survival value when living on edge of human habitat?
Who knows?
I once visited a chimp sanctuary in Kenya. One of the chimps walked upright all the time. It was because he had been confined in a very small cage since very small where he could only sit or stand erect. His bones were misshapen. It was comical until you learned the reason for it.