If you don't want to influence what he believes about 9/11, just tell his no one really knows what happened that day.

Then tell him some people think (then go on to explain whatever conspiracies you heard or believe), then tell him that other people think (explain the official story).

My son is only 6.5, but that's how I explain things to him that can't be proven without a shadow of doubt. Like you, I may hold my own personal opinions about certain subjects, but I want my son to make up his own mind when he is old enough to.