If you investigate the BBC website, you will find lots of information. This Revision guide summarises information which may help to make sense of things.
I like Dorling Kindersley books for background reading on specific topics. I can be expensive to buy a lot of these, so get them from your local library. We have bought a few though as they are such nice colourful books to look at, as well as containing lots of information.
Other things to do. Go to toy store and get Chemistry set. We have one at home which contains most of the common experiments which they start off doing. Things like trying to dissolve salt, sugar and flour in water. Perhaps, your son doesn't understand why he's doing this? Thinks that the answers are obvious? Well, of course they are, we have all been doing Chemistry from a very early age. What he is learning now, is how to experiment with a scientific mind and how to record what he has done and his results in a scientific way, so that someone else could follow his instructions and repeat his results. He is NOT learning that salt dissolves more quickly in warm water, as he already knows that.
In Physics as well, he will be being told things he already knows. What he is being taught, is how to look at these things in a scientific way, and how to recognise patterns and rules. How to think about what might be causing things to happen. Find the story of Isaac Newton and the apple for him!
The problem today, is that so much Science teaching is being done by non-specialists. Hopefully, at a Grammar School this is not the case. Watch out for this though, as sometimes the teachers have got the wrong end of the stick, and that is why the pupils are confused about why they are doing things.