I would immediately discourage the scaredy-cat language and stop any family member from teasing him.
There's a lot of people that have anxiety about rollercoasters or any attraction for that matter. Whether they're generally scared or just have anxiety about the height, drop, vehicle, latching mechanisms, seats, ears popping, darkness, ghosts, etc.
Attractions are overstimulating by design and it's perfectly normal.
One thing you could do is have him look up all the POVs of the attractions so he knows what he's getting into.
Universal tends to make rides that have less flexibility in terms of height, weight and overall size. I'm broad, tall and somewhat overweight.
When a new attraction opens, I always make sure to check out ride POVs on YouTube before I even attempt to go on them. Makes me feel much more comfortable and secure.