Any help with the below would be well appreciated!
It is an American live action film. I watched it on TV in the UK in the 80s. I don't think it had a theatre release. There were no famous actors that I remember.
It begins with a women walking with her son in a city. Someone walks up and pepper sprays her, when she recovers her son is missing.
The film cuts to her reading, now wearing glasses. There is a knock at the door and it is her grown son.
She is married (not to the father) and her husband does not trust the new boy (lets call him Stuart as it is easier than writing the boy but it is not his name in the film).
Stuart and his mother's husband are talking when he is in the bath and Stuart catches a radio that is falling into the water and thus saves him. He also wins the husband's trust by showing him a birthmark.
Later they are at a cabin next to a frozen river where both Stuart and the husband are on the ice. The husband realises that Stuart is a fake, so Stuart pushes him through a hole in the ice and drowns him.
A police officer arrives and Stuart claims his mother is drunk and hysterical and her husband died several years ago.
His mother suspects something is wrong and asks why Stuart had not told the police officer about her husband dying.
He eventually draws a gun and forces her to bake cookies, his birthmark wipes off. He explains that he was kidnapped the same as her son and that they were all tortured etc. Someone tried escaping once and they could hear him screaming for weeks.
However, I am wracking my brains to remember the ending but I can't! Nor can I remember Stuart's motivation for what he is doing!