After installing nvm using brew, create ~/.nvm directory:
$ mkdir ~/.nvm
and add following lines into ~/.bash_profile:
. $(brew --prefix nvm)/nvm.sh
Open a new terminal and this time it should not print any warning message.
Check that nvm is working by executing nvm --version command.
After that, install/reinstall NodeJS using nvm install node && nvm alias default node.
Hope this is helpfull!!