Hi, I know I've come across this late, but thought I might be able to offer some support, if not help.
Your ds sounds exactly like mine (currently 5, 6 next April) he is bright, articulate, intelligent and very interested in everything but just not that keen on reading or writing. Your comment about it not being a question of whether he CAN do the work but rather that he doesn't want to, really struck a chord with me.
On top of that, he was very, very ill last December and as he was already not doing that well in class, when he came back after a hospital stay followed by the the holidays, he had literally forgotten everything he had learned, couldn't even write his own name or read easy 3 letter words, as a result he was put on the special needs register, given an IEP (Individual Education Plan) and was 'supposed to' get additional help in class.
We found that despite all this, he still didn't make much progress and I decided to do some work with him at home and try to make it more interesting and focussed on what he was interested in.
First of all I bought him a couple of sets of more interesting phonic reading books from red house books Biff and Kipper just aren't that interesting to a 5 year old boy imho. I also bought a couple of cheap exercise books from Tesco for him to practise his writing.
Each day I wrote out his name, plus four words from the Reception high frequency word list in his exercise book, starting with the short, easy ones. After school he would come home, have a snack and then copy his name and the four words, then he could choose up to four more words on any topic he was interested in for me to write and him to copy. I sat with him each time and helped with letter formation and size/position of letters etc.
It was tough getting him intererested at first, but his writing definitely improved and then he started to get interested enough to think of the words he 'wanted' to write the next day. By the end of the summer term he had written all the high frequency words three times (by this time I was letting him help me chose which words to write and this also helped his reading skills) and I no longer needed to write his name for him to copy, he could just write it himself at the top of each page.
I also encouraged him to draw pictures of whatever he wanted and then write a 'title' word at the top and his name at the bottom of each one.
When we had his learning review they said he had met and exceeded his targets and was now ahead of quite a few of the other children in his class.
Reading has been a tougher nut to crack, he wants to be able to read, but finds it hard work and generally is quite 'lazy' for want of a better word, with regards to the effort he needs to put in. However, I bought him this set of phonic reading books at the start of the summer holidays and he has really enjoyed them. They are really well structured, introduce the key words and sounds etc before each story and the stories are not too long so he doesn't lose interest half way through. They also recap the important words at the end of each story to reinforce the learning.
We are definitely not there yet with the reading, but I find just 20 mins a day with him reading one day, writing the next has made a massive difference to his skills and with each success comes greater motivation. Also, choosing things that he is interested in to read/write about has really helped to keep him motivated.
Sorry to ramble on, like you said, its early to be worrying about his reading and writing skills - they will come when he is ready, but if you did want to help him along a bit I hope I have said something useful in here somewhere.