Unfortunately some people are just narrow minded. You did speak up and that was good but it may have become ugly if you shamed him in public. If it comes naturally and you get the opportunity you might ask him why he thinks it is stupid to want to die. Leave it an open question and if he seems open to it give a little information. Explain a little of how depression is a disease and how it can sometimes be inherited. If he doesn't believe in science you just have to let it go.
I think you can let it go. You didn't do anything wrong.