If he lived in Ystrad Mynach, this would come under Caerphilly County Council now, and their burials are online.
If you call 01443 815588, and ask to be put through to Burials, they will look this up for you.
The death would have been registered in Merthyr Tydfil, but I wouldn't have thought he'd have been buried there if he lived in Ystrad Mynach (Gelligaer). There are a few Churchyards around Ystrad.
If you find it, and can't get there before you go home, I'd be happy to photograph the headstone for you, if there is one.