Gail's right; ask your ped. about medicines, or maybe SeaBands. For practicalities, I can suggest a few things, though. (DS had reflux, and only recently outgrew it. I'm an old hand at dealing with airsickness.)
Be sure that you have a full change of clothing, including two shirts that will fit your child, and one shirt that will fit you (or whoever is sitting next to him.) Bring a wet washcloth in a ziploc, a bit of liquid detergent in a small bottle, and also extra large ziplocs that you can put soiled clothing in if necessary. Also bring some *diluted* mouthwash in another small bottle, plus his toothbrush, so that he can clean up properly if he does get sick; full-strength mouthwash can be too much for you if you've just been sick. Be sure to carry a good-sized sports bottle of water onto the aircraft for him; you don't want to be dependent on the FA bringing water if he needs to take medication quickly or rinse out his mouth. (The running water on a plane is not potable, you need bottled.)
Obviously, be sure that his seat has bags in the pocket, sometimes there are none there. If they are missing, ask the FA to give you some replacements just in case. FYI, at 12 this isn't likely to be an issue, but for those with small kids with this problem ... carry a little beach pail onboard. Little kids usually can't handle getting sick neatly into a paper bag.
The most likely time for airsickness to strike, BTW, is just after landing, as the plane taxis to the gate. Don't decide that being back on the ground means that the danger has passed; keep your eye on him until he is actually walking off the plane.