Is there anything riding on the results because option one is to do nothing and let the results speak for themselves.
More pro actively:
Devise a reasonable revision plan together.
Check what directions the teachers have given - what to revise, types of question etc.
Focus on interactive revision - that in itself reduces day dreaming etc:
Making revision notes in form of bullet points or diagrams etc.
Look, cover, write ( whiteboards useful) anything to be committed to memory.
Practice questions: text books often have questions at the end of each chapter or section.
Spilt sessions into short chunks ( 20 mins?) with brief breaks ( 3 mins ? ) built in involving standing up, stretching, getting a drink...and a 'treat' at the end ( eg 20 mins on x box but involving fresh air if possible!)
At this point there is a very long road ahead so from experience I would say don't put on too much pressure. The most important thing over the next few years is to discover what works for him. He needs to realise that sitting in front of a text book day dreaming rarely helps
but its good to try out different ways at this stage.