Definitely this sounds like nerves/excitement rather than true motion sickness. He will by now be more nervous about being sick than anything else. Calm reassurance from you, and doing some of the lower stress things suggested earlier should help.
What are his motivations for learning to fly? If for leisure only (ie not for a commercial career) then making sure the lessons are fun, so he can learn to relax, is important. He wants to do it, and you don't want to lose a student. He may be putting too much pressure on himself at this stage (pressure is your job, later on) in his eagerness to make progress. If he has financial constraints that are driving you both to work really hard its worth remembering that people don't learn well when under stress - relaxing a bit will help and accelerate the learning process.
Best of luck, and let us know what happens!
H