Oh man, that blurring of the boundaries is great; you really show how Remus has been trying to compartmentalise it all and that the fantasy crosses into an unsafe area.
his son's amateur yet intuitive hands on his prick and bollocks
The use of "son" or "dad". Love it. It's, again, hitting that awkward place in loving incest where there's a question about what they're doing but a reminder of the affection, and I squee a little inside :).