I think I agree with the posters who have said not to bother too much with your little fellow acquiring reading skills. Reading to him and encouraging him to think of writing as a code is always a good way of sparking curiosity.
In the system my DCs are in (not UK) the teaching of reading is lighthearted and fun in reception and later years with the main reading taught once the child has reached 7.
What reception children are taught is to be curious, and to think about stories, how they are constructed - characters, scene, tension, resolution. To appreciate rhymes and melody in the languages they learn (bi / tri-lingual).
And they do a lot of mathematics in the early years; not just arithmetic, but 3D shapes, sorting, volume, symmetry, pictorial representations, fractions etc. all for fun, and part of games at 5 and 6 years old.
Who wants a 4 year old reading/ writing anyway?! So often the books they end up reading have age-inappropriate content.
At this early age it's far far better to get them to use their imaginations, shove paint around a page, experience the pleasure of making music, have tactile experiences, do a lot of running around, learning to swim / cycle / play in a team. Plenty of time for them to be bookworms later 
Have a great summer with your little fellow OP.