Now, I'm not familiar with the speaker, or his message, beyond what's in the article. I don't think the mayor has any business asking these people to change a guest speaker they've arranged.

If one doesn't like the message of this speaker, debate him. Refute his points. He's going to make it super-easy and lay them all out for everyone to see. It is through the exercise, and embracing of free expression that ideas are brought into the light, examined, and measured on their merit. If he is silenced, his message does not disappear, it simply goes underground, and one loses the ability to refute it's points.