It's not a violation of the 1st amendment, because the school didn't say he couldn't become a student and talk to all of his buddies on campus about that.

The government didn't say he can't go setup his own booth and speak about it.

The school simply decided that it wasn't appropriate for their atmosphere and that they would not be paying him -- and then not be sponsoring the event in any manner -- due to the damage it would most certainly cause the school in future enrollments.

1st amendment rights violation would be if the government said he can't speak about that anywhere, or if the school told a student he couldn't make a sign depicting 9/11 in a positive light, it does not mean the school can't decide what its speakers can or cannot say -- and it doesn't mean that if those speakers that are set to come on school property and speak decide to speak on a subject that is disliked or inappropriate in the schools term, the school can't tell him he isn't welcome.