I know you've probably done all this.
Talk to every relative you can, even of your generation, they may have been told something even the smallest bit of info will sometimes allow you to eliminate some of those on the MRI's. Most importantly have any of them got his medals ? If you can track them down you've got him.
I'm assuming you've looked for his service records and found none.
The two most likley candidates from the MIC's will be the two HLI men, it was the local regiment of Glasgow, more Glaswegians joined that regiment, than any other single unit or regiment.
There are also the Electoral rolls hopefully he may be listed as an absentee voter.
If his service records are missing you are never going to fully trace his service, but with a regimental no. you can identify the battalion he served or at least one of them. with his no. if you find it from whatever source i.e as suggested did he have children born during the war - you can identify his MIC, this will list any transfers and subsequent no.s he may have been issued.
Once you've identified him, and his regiments or units posted to you can track his movements through the regimental war diaries, and therefore the actions he took part in.
Hope that helps
Stanfast