the guy is deluded. so he won't care how clearly its explained to him. the level of quality as a person is the use of calling someone the R word in an effort to insult someone who doubts his statement.
Sadly, one can never have a reasonable conversation with someone who finds calling it fitting to insult someone that way. saying that someone is mentally handicapped isn't really some sort of cool insult. you should thank God you aren't raising a child or have a sibling with any sort of serious health or mental challenge.