'How do I revise for GCSEs mum?' HELP!

[Spidermama]

DS has just revealed how far behind he is with this. I have been trying to get him to revise for ages. Today he has finally decided to start but he has no idea how. I assumed they learn this at school. There doesn't appear to be anything useful on the school's website. I'd really appreciate some ideas here please.

I suggested mind maps. Pick a subject, break it down into topics and then sections within topics.
Work out which he knows best and put a green sticky label on these. Any that he needs a little more work but understands the basics code yellow and anything he's struggled with code red.
Start with the red subjects as you can use your teachers knowledge for the next few weeks and then move onto yellow and green, recapping red nearer the time.
Use the methods above to help. There are YouTube videos for some subjects and examboards. Google can help with this.