Hi brigidmac - found out some information to take into account regarding the uniform question.
Summary: This may be Walter, but he's not wearing a standard issue RSF uniform in the shot.
About the style of photo:
1) It looks like a studio photograph where he wore various pieces of military uniform, a mix from different regiments. Here's a thread about another photo where the subject is wearing a mix of pieces that don't match a specific regiment:
https://www.rootschat.com/forum/index.php?topic=398213.0Why would they do this? It was tradition for soldiers to get a photo in uniform before they left to join their regiment. However, they often didn't have their uniforms yet. It's believed the studio lent pieces for the shot and the pieces didn't always come from one regiment or match the regiment the man was joining:
https://www.greatwarforum.org/topic/283249-studio-photos-ww1-loaned-uniform/About the uniform he's wearing:
1) The sporran worn in your photo has 3 white tassels on black. It is not the standard issue sporran for the RSF. Here's a thread on the standard RSF sporran which is 2 black tassels on white:
https://www.greatwarforum.org/topic/275204-royal-scots-fusiliers-sporran/The group that appears to have a sporran with white tassels on black is the Cameron Highlanders. There are also a few photos with this sporran that say it's Seaforth Highlanders, but the majority of Seaforth Highlanders photos have sporrans with black tassels on white. Neither group has 3 tassels in their sporran in any photo.
Here's a thread on the Cameron Highlander sporran. Way down the page there are photos of Gordon Highlanders sporting sporrans with 3 white tassels on black, quite like the one in yours. It may be a Gordon Hignlander sporran on your gent:
https://www.greatwarforum.org/topic/181095-guide-to-sporrans-continued/page/10/2) The next thing is the tartan. It's hard to see, but doesn't seem to match any of Seaforth Highlanders, Gordon Highlanders, or Cameron Highlanders, the ones that were possible matches on the sporran. It looks like it may match the Black Watch?
3) The jacket is quite big on him and it doesn't resemble any others documented for the RSF in WW1. Here's a good shot of a group of RSF in France 1917 where you can see the arms quite clearly. While some have insignias on the sleeves, they don't match the one on the sleeve in the photo. (a couple guys in the back row aren't RSF, they're easy to spot bc their hats don't match the others)
https://www.flickr.com/photos/14684508@N02/15198995352/in/album-72157647044333878/Happy to look more if you want me to try and dig up any other details or info!