Hi Auster - the main thing to know is that your ds isn't unusual. There are loads of us on here with dcs in the same boat and they'll all be dry one way or the other eventually!
Here's a thread I posted last October about this subject (ds is still not dry btw - he's now 5.4) and there are many more with lots of good advice.
www.eric.org.uk is worth a look and I would recommend getting along to your gp and see if you can get a referral to your local enuresis clinic. Different ages seem to have different minimum ages when they're willing to refer but our gp was willing to refer ds at 5 (we're still waiting for an appointment though).
I've bought This book recently which uses NLP techniques - I'm a bit skeptical, but we'll see...
Our dcs will be dry eventually, it just takes time, but I do think it's worth exploring options rather than just sitting back and waiting, while accepting that some just get there sooner than others.
There are also some things you can try to get into good habits - double voiding (trying for a wee immediately before getting ready for bed and then again before going to sleep), avoiding certain drinks (blackcurrant seems to be a factor for some children), making sure they drink plenty during the day to make sure the bladder stretches and gets used to holding on etc.