Hi Gordon,
That's a piece of good news!
If you have a look here:
http://www.rootschat.com/forum/index.php/topic,495487.0.html you'll see an aerial view of St. Peter's with the 'Common Ground' Sections marked with a small red 'c'.
I believe (although I may be wrong), that although you'd be unable to locate a single in these sections, there will be Register of Burials in Common Ground. (I have looked at the one for St. Kentigern's Cemetery held by the Mitchell Library.) The lairs in Common Ground are as precise as every other privately held lair, so there will be a 'plan' somewhere.
When you think about it, there would have to be record of some sort - otherwise how would the cemetery staff have known exactly where any past/future burials would be!
If you can, go along to St. Peter's during the week when the wee cemetery office is open. The chap there is really helpful and I'm pretty sure he'd be only to happy to help if he can.
If details of the exact lair can be found, usually one of the staff will 'pace it out' for you.
I wish you the best of luck finding your wee brother.
Anne