Alexander "Alex" Stewart was the son of Alistair Gordon Lethbridge-Stewart. He died in 1979 at the age of ten. Lethbridge-Stewart, many years after his death, left flowers at the grave of his son. (NOTVALID: The Dark Dimension)
Alternative timeline
In an alternative timeline where the Fourth Doctor hadn't died and regenerated after falling from the Pharos Project and Ace hadn't met the Seventh Doctor, Alex became the boyfriend of "Dorothy." The couple had several children. Alex got kidnapped and was later possessed by the Creature who was responsible for this change in the timeline. Alex died when he was released from the Creature's control. (NOTVALID: The Dark Dimension)
Behind the scenes
The complete synopses of The Dark Dimension can be read in The Nth Doctor by Jean-Marc Lofficier.