"The Imposter" (2012) by Bart Layton, a documentary about a French imposter Frédéric Bourdin and the family of a missing child Nicholas Barclay.
Chameleon" (2010) by Jean-Paul Salomé, which I talked about it on my blog.
The whole story is very disturbing, you know your own child, you know when the person is a stranger. The child do not change much around the age of 14-17. So you know.
They are the only one know the truth what had happened to Nicholas.
Synopsis: A documentary centered on a young Frenchman who convinces a grieving Texas family that he is their 16-year-old son who went missing for 3 years.
Official Site: http://imposterfilm.com/
Recommended on DVD.