Fluxy, I really feel for you; I was nearly driven insane by dd1 doing this. Has he been doing it for a long time, or is it something that has started recently?
Dd1 started doing this (she is 4.5) a couple of months ago and I got so beaten down I ended up calling the HV, who recommended seeing one of the GPs at my practice - his own dcs had done the same thing. I had tried all kinds of stuff with dd, and had ended up letting her into my bed just so we could all get some sleep.
Doctor prescribed a mild sedative - an antihistamine, I think - but it was useless. Other suggestions were doing something to break the cycle - like having her sleep over at a friend or relative's house. It's a habit really, and they can break out of it - it's just finding out how.
Dd1 stopped for no real reason after about eight weeks - she still wakes up once or twice, but is nothing like as bad. I wish I could tell you what did the trick, but I can't really. But I can tell you that your HV, and your GP, should take this seriously if it persists to the point where you are all completely exhausted, so give your HV a ring if you have a good relationship with her.
Long ramble! Hope it's a little bit helpful.