Hi Sandy,
A first trip to Disney World promises lots of treasured memories. Its super smart of you to get a feel for attractions that may frighten younger kids.
Its pretty dark in the Haunted Mansion from beginning to end. In the preboarding room, there are a few seconds that it is totally dark before the doors open to the boarding area. You sit in a moving doom buggy during the 7 minute & 30 second ride. In one part you pass by a coffin with a corpse trying to get out (just see the hands). There is also a cemetery section that is a little loud & has heads jumping up (not out at you). Your doom buggy will tilt back just a little as you descend to the graveyard. My youngest was 4 1/2 when he first road it. Before we got in line, I explained to him that it would be dark, there would be times it was loud, that nothing would jump out at him or grab him & that nothing was real. I "talked" him through the first time pointing out silly things & he made it fine.
Pirates of the Caribbean is a little dark in some parts & brighter in others but for the most part is comical. There is a small 14 foot drop in total darkness near the beginning. When you pass through Davy Jones' image you'll see a skeleton & a moving crab on your left - the drop is just beyond that. I always hold onto my boys & laugh the whole way down so they don't get scared. I think your son will love this ride.
The weather is March is usually pretty nice with the average highs in the mid to high 70s & the lows in the mid-50s so bring a jacket for the evening. They usually get a little over 3 inches of rain during the month of March. I always bring rain gear to central Florida no matter what time of year I'm visiting. A couple of days before your trip, go to
www.weather.com & check the current forecast. Wishing you many magical memories :-) Allison