I am thinking that, since he isn't young enough to know what ghosts are yet that he won't be scared, but I'm a first time mom and totally clueless as to how he might react.
You have probably figured out the difference right there.
Many infants and toddlers are not afraid of the dark rides, as long as they are being held by someone they care about. They might not like the dark, but as long as someone is nearby to comfort them, it doesn't bother them. They are not really afraid because, they don't really know (for example) that the Evil Queen in Snow White is scary, but the 7 Dwarves are not.
The older toddlers or pre-schoolers do have some idea of those things and they start to have an imagination about what those 'scary' things could do. That's when they start being scared in rides.
Sometimes it's on rides that
do have scary things, like Snow White and being afraid of the Evil Queen (or the Heffalumps in Winnie the Pooh if they haven't seen the movie that includes a 'good' Heffalump). Sometimes, for those little older ones, it's scary if it's dark because they can imagine things that might be in the dark. Also, some things (like the 'pop-up' ghosts in the graveyard scenes at Haunted Mansion) might seem funny to a toddler who likes Jack-in-the-Box toys, but might be terrifying to an older toddler or child who is not quite sure whether or not those are
real ghosts.
I remember my older DD as a 5yr old was afraid on Haunted Mansion of those and of the last scene where a 'ghost' is in your car with you and the narrator says it will follow you home.
And, my nephew at 5 was afraid of Pirates of the Caribbean because at the beginning it says, "Dead men tell no tales." He understood the language and that was enough for him to be scared.
That's also why you will see posts from people who took a younger child on rides and the child was not scared of anything, but when they were older and went on the same rides, they were scared.
I'd look more for his general reaction to dark - is he OK with being in the dark if you are holding him?
Also, Spaceship Earth and Where's Nemo don't have anything that pops out at you or happens suddenly. So, I think they would be a better choice for a first try than Haunted Mansion, which has a lot of those. Or Pirates, where it looks like you are going thru a wall of water and you go down a small falls in the complete dark.