gravitarian
gra·vi·tar·ian
/gra-və-ˈter-ē-ən/
noun
a person of serious bearing who eschews frivolity; not to be confused with a "wet blanket"
/gra-və-ˈter-ē-ən/
noun
a person of serious bearing who eschews frivolity; not to be confused with a "wet blanket"
He doesn't participate in hijinks because he's a gravitarian.