This fleeting attention span could be attributed to his disrupted visual processing (one form of Sensory Processing Disorder) as he was not able to filter out unnecessary visual information (sensation) and focus on the task at hand. All visual sensations (images, light, faces etc) seem equally important and hence he finds it determine what is relevant and what is not for that particular situation. We observe it as distractibility.
A lot of children are aware of location and what is expected of them there. For e.g. they might really hold it together at school, where they know their freedom is limited and may not manifest many of their sensory integration problems at school. They will, however, fall apart when they get home. They know they’re in a safe environment and the impact and stress of their day may come out.
The child may be constantly on the move or may be slow to activate and fatigue easily. Again, some children may fluctuate from one extreme to the other.