What helped our kids are a motion sickness wristband and stocking the boat with ginger cookies and ginger ale. Seasickness is not something you can control through will alone.
Ginger helps, as does acupressure and staying on deck looking at the horizon. Can he see over the dodger/cabin while seated in the cockpit?
In cockpit while seated he cannot see over dodger. Really need to keep the dodger up as sailing is in Hawaii and off shore- otherwise wave spray will be intense. Dodger is very small. He can see out the sides ok and to the front around the dodger. I get him to stand when conditions permit so that he can see around and to horizon. He always wears harness and life jacket. I try to get him involved in the sailing as much as possible to get his mind off sea sickness thoughts. Sailing is usally pretty intense as winds here are normally 25 knots with seas at 6 foot minimum. I try to pick the best days for him and only fly a reefed down jib (no main) to reduce boat heel and speed. I will try the ginger. Also taking a friend might help.
Thanks for advice.