We visited Universal last week with a 13, 16 and 18 year old. None of the kids would go through the Haunted House (or whatever that ride--you actually walk--is called). My husband and I went in it but we weren't in the front of the group so we weren't jumped out at or anything. I thought it was really fun but I LOVE horror movies. There was a section where a mummy or something reaches for you but since we weren't at the front of the line, I knew to watch for it so I took my time entering that section and then ran through (my husband couldn't stop laughing at me for that).
It could be creepy if there were only two of you going through it and you had somehow scared yourself before entering. I just had the mindset that it was all fake and they weren't allowed to touch me and I was fine. I don't think your son would like it though.
As for the Mummy, that is a roller coaster and it is fairly short. It goes forward, stops, and then runs backwards. All, except the 18 yr old, enjoyed that ride and wanted to ride it again but we ran out of time. I think water drips on you and air is sprayed on your feet to simulate the scarabs but I don't recall anything else that was creepy on that ride.
Teminator 3 states it isn't for young kids but we didn't have time to go on that ride.
Our favorite (although my husband couldn't ride it because he can't handle many simulator rides) was the Simpson's. We had a blast on the ride! We also enjoyed the backlot tour and the Jurassic Park ride. I also found the sound stage tour interesting.
Enjoy if you end up visiting!