Support for the loopy 9/11 documentary “Loose Change” - which argues that the World Trade Center terror attack was part of a secret U.S. government conspiracy - is quickly losing steam.

A source tells us Charlie Sheen “is having second thoughts” about being involved in an updated version of the flick, which has a huge following on YouTube. As Page Six reported in March, Sheen had agreed to narrate the ridiculous flick, presumably to give it some needed Hollywood sizzle. ...