I have no idea if this would be appropriate for your seven-year old, so you and your doc should decide, I'm not a doc...
....but for my kid (he's 11 now, but sailing with me since 4 or 5), after talking with his doc, we give him a little Dramamine.
The side affects can make him (or anyone) sleepy, but what we do is cut it in quarters by breaking a pill in half, then breaking it again. At that small dose, there does not seem to be any side effects that he can notice, but it takes the edge of the motion.
Much fun ensues...
As others have said, he's not choosing to feel sick and probably isn't faking it. Some folks are more susceptible than others. A tiny bit of the right over the counter solution (if it's appropriate for him) might turn on the fun.
If all else fails, I might give it a try. Making him a little sleepy would probably be a good thing- he is a little high strung!