Stupidity: Wes is characterized as one of the more careful characters early on, buying both a stun gun and pepper spray. When he comes home after the meeting where the dangers of Ghostface were further laid out, he proceeds to set both his weapons down and takes a shower after his mom tells him he'll be alone in the house, which is exactly how Tara and countless victims of previous sprees were attacked. The fact he even lived through his shower is lucky enough.
Stupidity: Judy Hicks, a trained cop and sheriff with over a decade of experience, does not carry a back-up holstered gun on her when she is off-duty, one day after her son's close friend is stabbed seven times near to death. A trained police officer/sheriff would also question why the killer would tell her they are about to kill her son, before they do it, if not to bait them and lure them into a trap. (00:45:58 - 00:46:30)
Answer: As far as I can tell, It's never stated in the movie how he found out about that. Maybe Sam told him off screen when he "learned " that Billy Loomis was Sam's biological father.