I would get him to plan a schedule, so he can see there will be still time for gaming and watching sci-fi.
You could let him pick what he wants to do, as long as he includes some things academic in the mix, of whatever takes his interest. For instance: Learning about plants and insects; astronomy, early civilisations, clouds and weather systems, etc.
For maths, without going ahead in the curriculum, the junior maths challenge section of the UKMT site has lots of puzzle type maths questions. It is aimed at years 7 to 9, but has a range of difficulty in the questions.
There is also the Primary Maths Challenge, which also has past papers and is aimed at the final years of primary.
Khan Academy is worth a look at. It also has a coding course, along with many other subjects.
Do not worry if he does not stick to his schedule, just so long as most days it gives him the structure to get on with something.