The Register of Soldiers effects reports that he died in No 3 Canadian Stationary Hospital which was located in the Citadel, just outside Doullens. The Citadel is very old and has a mixed history. On the night of 29/30 May 1918 it was bombed by the German Air Force during their Spring Offensive.
The operating theatre was destroyed by a direct hit killing those within and setting the building alight.
Two surgeons, three Nursing Sisters, ten officer patients and a number of personnel and patients were killed whilst a number of others were injured.
A number of those killed including some of the nursing staff are buried in Plot III of the Bagneux Cemetery at Gézaincourt - which is about a five minute drive away from the citadel.
Following the war the buildings became used as a home for delinquent girls until the invading Nazis took over the structure as an internment camp for political prisoners. Towards the end of the war they considered setting up the Headquarters for the V1 and V2 missile bases on the site, but this came to nought.

The photos below were taken on my visit there on 20 June 2015.

He is buried in Doullens Communal Cemetery - extension 1 with the record entry:- YOUNG, Harry, Rifleman, 254313. Date of Death: 07/04/1918, Age: 21, London Regiment (First Surrey Rifles), 21st Bn. VI. D. 67.
Son of Mr. B. and Mrs. A. Young, of Dulwich, London.

Doullens was Marshal Foch's headquarters early in the First World War and the scene of the conference in March 1918, after which he assumed command of the Allied armies on the Western Front. From the summer of 1915 to March 1916, Doullens was a junction between the French Tenth Army on the Arras front and the Commonwealth Third Army on the Somme. The citadelle, overlooking the town from the south, was a French military hospital, and the railhead was used by both armies. In March 1916, Commonwealth forces succeeded the French on the Arras front and the 19th Casualty Clearing Station came to Doullens, followed by the 41st, the 35th and the 11th. By the end of 1916, these had given way to the 3rd Canadian Stationary Hospital (which stayed until June 1918) and the 2/1st Northumbrian Casualty Clearing Station.

From February 1916 to April 1918, these medical units continued to bury in the French extension (No 1) of the communal cemetery. In March and April 1918 the German advance and the desperate fighting on this front threw a severe strain on the Canadian Stationary Hospital. The extension was filled, and a second extension begun on the opposite side of the communal cemetery.

In May 1940, Doullens was bombed with Arras and Abbeville before being occupied by the Germans.

The COMMUNAL CEMETERY EXTENSION No 1 contains 1,335 Commonwealth burials of the First World War. There are also seven French and 13 German war graves from this period. Second World War burials number 35, more than half of them men of the Queen's Royal West Kents who died 20/21 May 1940.
We visited the cemetery on 20 June 2015 and put a poppy cross on his grave. His headstone inscribed:-