There are so many reasons at this, that a real talk with him would be necessary for know ' his ' personal ones. Since it's individual.
It can be he finds it somewhat ' silly ', or he thinks it's useless, unless he thinks it's to be ' weak ' to be like that....Etc Etc.. The best is to ask him the question, like : '' what he thinks about courtesy ? '' .. If he said something like : '' it's bullsh*t '' (for ex.), ask him what extent ? What to be courtesy represents for him ? ....