You could just be honest without being brutal and bring it all back to work. You are, after all, at work and trying to be a productive company. To have a productive company, you need good leadership and if your boss is going off-topic with random hoaxes, this is hurting their leadership and the way that the employees view him.
One day, just before you leave or he leaves, you should mention that you are worried some of the topics he randomly brings up are actually hurting his credibility with the staff -- he is becoming more of a friend with bizarre ideas that don't make much sense instead of a boss that people see as their leader.
Come to him as a good person that is looking out for his best interest. You know that he is just trying to be helpful and spread some good information he read, but it might not be coming off the way he sees it. Let him know that some of the things he brings up are silly and don't make much sense and just come off as distracting as they don't pertain to the work you do. Let him know that you want people to look up to him as their leader, but when he does some of this stuff, it doesn't show the best leadership. You look up to him as a leader and want the other employees to do the same.
Hopefully you can play to his strengths and he will see you as someone that has his back and see's the greater goals of the company. If he thinks that his hoaxes are not silly, you could have a couple of his hoaxes in mind that have been clearly debunked to offer a different perspective.