Right, first of all thanks for the advice.
I've cleaned his cage and put the dust bath in there permanently for now because it's something he loves. I had him out in his ball for a few minutes and then out with Cheech to free run in the room. He was sneaking up on her as normal but she was a little vicious towards him. He still doesn't seem to have touched his food but I gave him about three pieces of the alt. food I use as treats and he is eating hay and he is pooing although they are small. He was definitely more lively after the run, he's asleep now but they generally wake up about 3am and go to sleep about 7-10am. He seems to be getting marginally better every day which is why I'm not as worried as I could be, although I still am worried. It's the fact that he's not eating and he usually has such a big appetite. I will keep you posted.
Cheech doesn't mind what we do, she's so laid back. I can change her cage around, move her, whatever, but Chong is a little bit more touchy. He's currently asleep with his head poking out of his house which is better than before when he was right at the back.