Please stop trying to teach him, it's not necessary and it sounds stressful for you both.
Look at practical skills - can he dress and undress himself, including tricky buttons, zips, shoes etc?
Can he use a knife and fork and knows what is expected table manners at mealtimes, knows how to clear his plate etc?
Drink from an open cup without spilling? Pour from a larger jug into a cup?
Does he know how to recognise his own belongings in a pile of other people's?
Can he use the toilet totally independently (including a passable attempt at wiping his bum?) Is he confident using communal type toilets (e.g. public toilets)?
You can work on these things in a fun way totally stress free (positive reinforcement, games) and they will be useful to him. It doesn't matter if they aren't 100% but the closer you can get to these things being mastered will be helpful.
Remember that there will be kids coming into reception who have never been read to, whose parents don't engage them in conversation, who don't know how to count. He probably knows all of these things already. He is lucky. He won't be behind.