He could be the Henry Bristow who was the brother of Charles and Erle Bristow and who would have been around 29 or 30 years old in 1916.
Or he could be the Henry Bristow who was the brother of Richard Bristow and who would have been around 22 in 1916.
Alternatively this man may be unrelated to either of the two families mentioned above. Bristow was a very common name in
time (and is still common) and without further information at this point in
time it is difficult to pinpoint this man. Sussex