Hi Savvy57
I agree with PiedWagtail - I wouldn't worry about what other children are doing or where your DS is at just yet. Many children, especially if they've been at home haven't had some of these pre-school skills yet and I'm sure your DS will not be alone and will catch up very quickly.
If you haven't done much work on the alphabet or numbers to 10 or perhaps 20 - then gradually over the next few months (and there is plenty of time so this can be quite relaxed) start introducing saying the alphabet (there also are lots of alphabet books and counting books out there - so either buy some or visit yoru library). Probably the first place to start with writing is learning to write your name and recognise it - so perhaps that is something you can work on. Kids enjoy writing it in water on the patio, painting it in big letters on a big sheet of paper or writing it in chalks on the pavement. It doesn't just have to be pencil/ pen and paper.
I definitely agree with PiedWagtail the jolly phonics books are really useful to work with at home during YR.
Here on Mumsnet there is also alphablocks worksheets (under literacy) and a counting to 10 games (under numeracy) on the education pages: www.mumsnet.com/learning/learning-zone/learning-zone-introduction
If your DS is keen to get on with writing than go ahead an encourage that. But try to avoid writing in all capitals - so try using lower case and explain that the capital is a very special type of letter at the start of a proper names (like his name or the town/ city you live in).
For counting I'd highly recommend snakes and ladders - it helps get them counting up to 100. When that's secure you can play counting by 2s or 5s (just use one die and count up how many you roll x 2 - so if you roll 5 - you'd count by 5 twos.) Play backwards for subtraction practice.
You can count even and odd numbers by saying house numbers as you walk somewhere (just come back on the other side of the street).
Finally take a deep breath and remember that a lot of Year R is about settling into the formal and busy environment of school and is centred on learning through play. Obviously as your DS starts school you'll want him to start building reading, writing and maths skills - but it's a marathon not a sprint and is individual to each child. Focus on making learning fun, making sure you're aware how your DS is doing and what he needs to be working on and making sure you are building good studying habits (so regularly setting aside time for reading guided reading books, doing homeworks and taking advantage of other learning opportunities through clubs at school/ at weekends, etc...). Be informed about what the notional average student should be able to do by the end of Year R, but use it as a guide. You will be amazed how quickly he gets on, but the important thing in YR is settling in and enjoying school. Don't underestimate how important liking being at school and working at school can be long term!
HTH