Hallo Andy -
Only info I could come across readily was his entry in Stephen Cameron's book 'Belfast's Own', which I suspect you might have already read? It mentions his sister, a Mrs Leathim, but not his parents. Perhaps if you contacted Cameron via his publishers he might have more information - I've corresponded with him in the past and found him to be exceptionally prompt and helpful.
Hopefully someone else on the board can be of more assistance to you!
Regards,
Inger