Rosie, Becca, can I say I know exactly where you are coming from, my ds is 6 on Monday and is still not 100% there, however he is a lot better than he was, he is slowly growing out of it. Whats more his cousin and one of his uncles were just as bad, so it can happen.
Becca, if he finds the toilet boring, can you put something interesting in there for him to do, but which must stay in the toilet, a book, cheap handheld computer game, something he can sit on the toilet and do, but which he is only allowed to do when sat on/stood at the toilet?
There is a Bear in the Big Blue House book called "When You've Got to Go", which deals with various toilet problems, including going when you are in the middle of something. Published by Simon and Schuster, london, ISBN 0743429109. A nice story involving well known characters.
Another thing we have done is explained where wees and poos come from and why we have to do them. I happened across a great Usbourne book for kids which explains in detail (probably more than you or I would want, but the kids sem to love it) what happens using a diagram of the body with flaps to lift so you can see inside the body. I cannot find it at the moment (my ds loves it and has obviously had it somewhere) but will post the details when I do.
As for rewards, we have a sweetie tube (actually a CLEAN urine sample bottle, but anything of a similar size should work), everytime he does a wee on the toilet he gets to put one sweet in, a poo gets two sweets, both gets 3, but he has to remove 1 (wee), 2 (poo) or 3 (both) if he doesnt go to the toilet and does it in his pants/on the floor (or whatever can take his fancy - he will wet anywhere to get a reaction sometimes ). When the tube is full he gets to eat the sweets. I use dolly mixtures, and he gets about 10 sweets in the tube.
One piece of advice I got from the school nurse was to actually stop making it such a big issue and just treat it as any other behaviour you expect but dont always get, eating all his tea, putting his clothes in the wash etc. Dont go mad, just give a similar level reaction - then it doesn't give them the feedback they want (as long as it is behavioural). Oh and she also said it is more common than you think, it is just something ppl wont admit to!
Hope these may help, feel free to cat me if you need to let off steam, or just to chat
Leanne