Boisterous is a adjective that describes The noun manner this is the best adjective because Dr lanyon manner is described as boisterous and boisterous means roudey and excited so it sums up his old personality

When Stevenson says that lanyon went from rosy cheeked man to then going to pale do maybe lanyon never took care of his self

“His flesh had fallen away”

“Tokens of a swift physical decay”

Lanyon went from a rosy cheeked and enthusiastic with a boisterous manner Then he describes lanyon by saying that his appearance and his mental state has started to decay