I'm trying to remember what I know about his sons- I think they all initially moved to other countries, Twm lived in Brittany for a long time, another is in Canada or America, not sure about the third. Twm then moved back to Wales and 'kept the flame' (as the Telegraph article above puts it).
I've come across Twm a couple of times and have wondered what his relationship with Jan was like. I did look for Welsh language articles when JM died, and came across this one.
from BBC Cymru (it's in Welsh)
It was commissioned to be written about Jan Morris on International Wonen's Day, and Twm begins by writing this:
As I am her son, I was asked to write a piece about Jan Morris, the world famous author, for International Women's Day. Her contribution to the world since 1972 has been enormous, it's true, and there is no bigger fan than me!
But there is plenty of talk about Jan Morris, the world famous author at the moment, and the second volume of her reflection diary Thinking Again has just come off the press. And that or any of the pile of books she has written during her long career would hardly have happened if it wasn't for her partner, my mother, Elizabeth.
Elizabeth also deserves attention on Women's Day.