Nothing (in my opinion) is even remotely scary for a 9 year old during WDW's Halloween season, unless he is very, very easily frightened. If he can handle Scooby Doo type spookiness, he'll be fine.
During parties or after hours events they will play Halloween style songs, and have lighting and fog effects in certain areas of the park.
You can check
YouTube for videos of past Mickey's Not So Scary Halloween Party dates, or last year's Boo Bash After Hours if you're concerned. Both of those are hard ticket events that are not included in regular park admission.
Halloween is one of the most fun times at WDW - I hope you have an amazing trip with your family!!