A Londoner in a Scottish regiment?

[Rosinish]

On his marriage/baptism his middle name is STANLEY not SIDNEY

If his middle name was STANLEY...

"I wouldn’t have thought the mention of" SIDNEY "would have anything to do with it??"

Annie

Added...Knowing who you're looking for helps before posting wrong info. but I see you have edited your post without making it obvious (as a correction) for those of us looking for the original name of SIDNEY you posted

[Ladyhawk]

Here’s a similar cap to the one on your picture https://en.wikipedia.org/wiki/Service_Dress_(British_Army)#Service_Dress 
this Lieutenant's cap looks similar but has 3 dotted bands
his regt. Argyll and Sutherland Highlanders

It might be worth putting your picture on the armed forces resources board to see if someone with more knowledge can identify the uniform and/or regiment and add a link to this post so information is not duplicated.

[Maiden Stone]

Did he join up in 1914? I believe that after The Somme in 1916, when "Pals'" regiments suffered a high casualty rate, new recruits were shared out to regiments over a wide geographical area. Someone else may be able to confirm this.
There is/was a London-Scottish rugby club.
Did he have Scottish ancestry? Maybe he just liked the uniform.

[Kerby]

Did he join up in 1914? I believe that after The Somme in 1916, when "Pals'" regiments suffered a high casualty rate, new recruits were shared out to regiments over a wide geographical area. Someone else may be able to confirm this.
There is/was a London-Scottish rugby club.
Did he have Scottish ancestry? Maybe he just liked the uniform.

So far no Scottish connection at all.

[Ladyhawk]

Not that this info. helps with the regiment name but thought I'd post.......

Frederick Stanley Reeves
21 Mayola Road E5
Year:   1945
County or Borough:   Hackney
Ward or Division/Constituency:   Hackney
Street address:   Service Register Town Hall Annexe, Reading Lane Hackney
7th May 1945

Index letters meaning
W – War Worker
S - Seaman
No index letter printed the elector is a member of the Forces

There is NO index letter next to Frederick’s name

[Maiden Stone]

I know of some men in Lancashire and Yorkshire who joined an Irish regiment.

[jim1]

The Glengarry doesn't appear to be London Scottish. I can't see a Fredk. Reeves with or without the "s" in any Scottish Regt.
So either not him or he didn't go overseas.

[Rosinish]

Looks like The Gordon Highlanders?

https://www.pinterest.co.uk/pin/721350065286505276/

https://www.gordonhighlanders1914-18.co.uk/uniform.html

Annie

[Ladyhawk]

Looks like The Gordon Highlanders?

https://www.pinterest.co.uk/pin/721350065286505276/

Annie

Well spotted Annie 

Frederick was on the Service Register for 1945 perhaps one of these entries is him - sorry I don't have a subscription for F M P to look at this site 

F S Reeves 1941 – 1945
British Army Casualty Lists 1939 - 1945