I didn’t realize J.R had cancer on the show, but he was in some sort of nursing home at the beginning and the nurses said he had been depressed. Bobby was diagnosed with stomach tumor cancer, so that’s probably the character you are thinking of.
I watched Dallas back in the day, but don’t know that I was watching it regularly toward the end of the series run. I had forgotten so much that a friend had to catch me up on what was going on with the show when it went off the air.
It was good to see so many of the original actors, though they have aged as the rest of us have!