Pugsley: Are they dead?
Wednesday: Does it matter?
Gomez: Have no fear. The courts will decide. They say a man who represents himself has a fool for a client. Well, with God as my witness, I am that fool.
Gomez Addams: [admiring Morticia, who is sleeping.] Look at her. I would die for her. I would kill for her. Either way, what bliss!
Answer: He's seen pictures and paintings of young Fester and notices a resemblance. Like you said, no-one had seen him in 25 years, so all that was required was a passing similarity.
Brian Katcher