Sorry it's taken me ages to let you know how he is.
So Bruiser spent a couple of days in the vets and then they called my flatmate to take him home. During this time my boyfriend's mum was phoning in to find out how he was and letting me know. My flatmate said that the vet wanted to have a word with me about him when I got back from Tenerife.
I went in to see the vet on Thursday. Apparently there's the starts of spurs on the insides of Bruiser's teeth, which may be making it uncomfortable for him to eat hay, which then causes problems with his gut. They said they weren't really big enough to do anything about just yet and that, hopefully, if I get him eating more hay that will wear them back down again. He's due to have his vaccinations in March so they'll take another look at them then and if they're worse they'll file them down for me.
So now I've just got to try and find some hay that Bruiser actually enjoys eating! He'll nibble at the Timothy stuff I buy him, but doesn't eat much. He ignored the Timothy with Herbs that I bought once, so we've never tried that again. He likes the Pets at Home "Hay Cookies" better (I think they're kind of like the hay cubes, only round), I think because they're greener. And he gets to throw them around, which is just fun! Excel do "Natural grass" hay which is a lot greener than any other hays I've seen, so I think we'll be trying that next. Hopefully he'll like that!
Anyway, he's home now and much more like himself again. He's back to running round my room and getting into mischief rather than sitting in his cage corner ignoring the world. He was even flopped down OUTSIDE of his cage this morning, which he normally never does (he's too shy and nervous). His poops are slightly smaller than they used to be, but I'm seeing more of them so I think the same volume is coming out. He's scoffing his food again and back to begging for raisins so I think we'll be OK.