He is a little c***.
Luke is a badly behaved child and prone to destructive tantrums (rather like his father). For example in Series 1 Episode 3: The Fancy Dress Party Luke smashes a 'Wetherspoon's' window with his head as there were no sausage rolls at said party (which was for him).
As a result of his behaviour Becky and Steve dislike him.
Luke surived Leukemia, the party where he smashed the window was to celebrate his recovery.