If you think that he MUST read and write during the holiday, then think 'outside the box' of conventional books and worksheets.
Go to a museum / historic house / other place of interest. Print out the directions from Bing or Google Maps and get him to read them to you as you go (or teach him to map read, and get him to do it from a road atlas - adding up the distances to find out how far it is still to go would help his maths too). Get a children's trail or guidebook, and elect him as the gude, so he reads out relevant parts of the guidebook all the way round. many places have trails, so that would mean writing too. Buy a postcard in the gift shop, and ask him to write a postcard to someone - his best friend, granny or the cat, doesn't matter. make up oral stories as you go round 'just imagine if there was alittle boy who lived here, what would he do?' type of thing.
Think about online reading - is he interested in the Olympics? Could he find out some latest news, or something else about a sport he is interested in? make a poster showing the timings of the different stages in an event and write to fill in the winners?
Play lots of games with him. Many - Top Trumps, for example - involve reading and maths. Rummikub, Cluedo, Monopoly ... get him to read the rules, and the cards, etc etc.
Organise for him to do the shopping e.g. for a picnic - make a list, get him to choose everything and work out whether he has enough money for it - he can add a treat if he has enough left over but he has to know that before he reaches the till...
Or learn a new skill together. My son is currently on his second Lego animation - he plots them, takes the photos, edits them, adds dialogue (which he writes down for himself and others to read) and sound effects. Windows Moviemaker is free, there are ven better options on a Mac.
So much can be learned outside school, and it is so much more engaging, motivating, experiential than is possible in school. Why waste time doing 'school things' when you could be learning so much better and so much more??
And read him a bedtime story every night. Or read one page, and get him to suggest how the story might continue, or talk about the characters, or whatever.....