Go to
www.scotlandspeople.gov.uk, invest in a few credits and use some of them to find his birth certificate. It will tell you the names of his parents, including his mother's maiden surname, and when and where his parents were married.
Search using his mother's maiden surname to see if he had any siblings.
Use this information to find him with his parents in the 1921 census.
There is a death of John Anderson, mother's maiden surname Scott, in Glasgow in 2009, aged 89. This may be him but both Mackay and Scott are very common names, so you'd be safer to start with the birth and work from there.