Hi there,
Just a couple of points on this...
Do you know which necropolis he is buried in for definite? The Glasgow Necropolis is the one usually referred to as just 'the necropolis', which is beside the Cathedral, but there are 3 others: the Eastern Necropolis, the Western Necropolis and the Southern Necropolis. So if you can't find him in the first, it may well be that he is in a different one.
Secondly, the address you have been given is for Glasgow City Council, who I believe will charge heavily for doing any work and having to search if you don't know the exact location and reference for the grave you are looking for.
You might be better getting in touch with the Glasgow and West of Scotland FHS in the first instance, who are working on transcribing some of the inscriptions of Glasgow burial grounds at the moment, to see what info they can give you.
http://www.gwsfhs.org.uk/Cheers,
Clare