Hi Allison!
You can browse through attractions by recommended ages(on the left menu)
here. This is just a guide, though, since all children are different. My 5 year old son, for example, loves Stitch's Great Escape, an attraction that while not exactly a thrill ride, I have noticed is not always favored by little ones. On the other hand, I can not convince my son to come along with me on Dinosaur (in Animal Kingdom); he simply refuses to go on it. You might find that your child likes rides that involve drops or are fast, but he may not like rides in the dark, even if tame in comparision to other attractions.
You may also want to obtain a rider switch ticket to try out a ride for yourself before bringing your little man on board with you. This will also allow you to assess the attraction for yourself, while also allowing you to prepare him by giving him the scoop beforehand. That might give him more confidence and help him decide if he'd like to experience it for himself.
The Barnstormer at Goofy’s Wiseacre Farm is a great "first coaster", too. My own sons have not yet graduated completely from this one onto the big thrill rides. One is not yet tall enough (but eager to), while the other is tall enough but less willingly to repeat riding attractions such as Big Thunder Mountain Railroad; and not ready to give my personal favorite, Space Mountain, a try yet. However, an attraction such as The Barnstormer, and perhaps even others like Pirates of the Caribbean and The Haunted Mansion (since they are dark rides), might provide indications of how likely your son will enjoy the "bigger" rides.
Hope this helps! Have a magical vacation!