True motion sickness is a condition of the inner ear, it is real and not something imagined. He may or may not grow to adapt to it.
However you and your crew may just have upset stomachs (sp), try and avoid foods that increase acid production; fried foods, animal fats, citrus juice,... and take some tums before leaving dock.
1970 Havsfidra 20 by Fisksatra
On the Delaware River at Fox Grove Marina Essington PA