Does your child tend to get car sick? If not, I would say that the chances are pretty low. I am a former FA and I rarely ran into problems with children being air sick. The key is not to let your child get to warm. If there are any turbulence and he's warm he is more likely to get sick. He is also more likely to get sick if he is on an empty stomach, so make sure you have given him a meal before the flight and that you have plenty of snacks for the flight. Also keep him hydrated. If you have any major concerns ask your Ped for a recommendation for using an anti-nausea med like dramamine. Take it with you and if you have any problems you can give it to him, just keep in mind it will make him drowsy. One more suggestion if you act relaxed about him going and emphasize the fun of his first trip he will more then likely be just fine. Don't forget to ask for some wings and a first flight certificate from the flight crew (assuming most airlines still have them). It will make a great keepsake for the baby book.