score:20
Here's the exact description of the photo, according to Hitlerpages.com:
Hitler and his fellow-soldiers in Fournes en Weppes, April 1915.
First row: Adolf Hitler, Balthasar Brandmayer, Anton Bachmann, Foxl, Max Mund. Second row: Ernst Schmidt, Johann Sperl, Jakob WeiΓ, Karl TiefenbΓΆck.
This way, your man is called Balthasar Brandmayer. But it turns out that he's not just "some random soldier sitting next to Hitler" as we could think.
In fact, he's an author of a book called "Two messengers" which was released in 1932. It speaks about the times of his war service and frequently refers to Hitler. I don't know if it was translated into English, but...
I recommend you the linked book "Corporal Hitler and the Great War 1914-1918, The List Regiment" by John F. Williams, as there's a lot about Brandmayer, with quotes from his memoirs. Unfortunately many important pages are not included in the online preview, but there's still a lot to read about him.
Below there's another photo from the same session, with Brandmayer lying at the grass.
If any of you wonder, the dog's name is Foxl (Fochsl).