Try combining visuals while listening to the language as much as possible, so that he associates new words directly with pictures or video rather than English words. There are websites that help you do this. The BBC website, in particular, has some good resources with the advantage that you can listen to the examples as well as reading them. What's more, the ability to listen to a native speaker means that you are learning from a perfect model. The BBC's KS2 "Roller" site http://www.bbc.co.uk/schools/primaryfrench/
has lots of games and other fun stuff for learning the basics in French. He could also move on to http://www.bbc.co.uk/schools/gcsebitesize/french/
to practice individual topics as they are covered in class.
Songs are also a good way of memorizing words and practising pronunciation, though probably better in a group situation - unless your DC is happy to sing along to his PC!
I sympathize with having to learn two similar languages - I could never get on with Spanish after studying Italian for a year!