There are a number of things that you can do to try to head off motion sickness. Simple things, such as not having a full stomach, can limit the symptoms (your child shouldn't eat for at least two to three hours before going on any rides). Avoiding dairy products and foods high in protein, calories, or salt appears to have some benefit. Limiting the amount of movement of the head appears to help some people. Have your son keep his head pressed back into the headrest of the ride that he is on.
Slow, deep breathing may help prevent the symptoms of motion sickness. Begin deep breathing five minutes before the ride and continue it, if possible, during the ride.
There are also over-the-counter medications available in your drugstore that can limit the symptoms of motion sickness. However, for an optional event such as an amusement park ride which is over in a relatively short period of time, I don't recommend taking medication. Almost all of the medications cause drowsiness, and it's not worth having that side effect for the whole day. Talk with your son's doctor if you think a medication is necessary.
If these suggestions don't help, then the bottom line is that you and your son will just have to decide if the fun of the amusement park ride is worth the throwing up when he gets off. Good luck!