Thanks everyone for your responses. He's starting to settle down a bit more now, he actually started eating pellets and grains for the first time in the morning of our hottest day for a while - not really sure why that gave him the kick he needed, but it did. He's also learnt to properly drink (not sure why I had to teach a 9 month old rabbit how to drink, but it's been 3 weeks of frustration) and rather than me having to drop drips of water down his nose into his mouth, he's now learnt to use the drinker. All around things are happier here, and he's eating a lot more grass too. I don't think it'll be long before he starts putting on some weight.
Vanessa, I did think I might lose him a couple of times over the last week! I've got a white cloth draped over a half of the hutch and every few hours I would go out and hose it down, and hose him down too if he was looking particularly droopy. He absolutely hated it, but I do think that it kept him alive. I tried the ice block thing, but he really took offense to it and kept attacking it and pushing it away from him. I am lucky enough (!!??) to be unemployed at the moment so I was able to go out every few hours and check on him. If I wasn't able to, then yeah, I think misters or a small sprinkler on the hutch all day would do the trick. Maybe something like those small sprinklers you have on an irrigation system. Not too much water would be wasted then. I have heard that too much lettuce can give them the runs, I've been keeping an eye on it, but I've had no problems with that so far. My biggest concern is that he was filling up on lettuce which is basically crunchy water, and not getting any nutrients or energy. I've dropped the lettuce ration down now, and added more herbs, not that he does more than taste them, which I think encourages him to eat more grass the the pellets. I'm at Semaphore, so we get some light sea breezes to cool things down.