Thank you all for your replies and well wishes, but I'm afraid to say that Boris did not make it. I accidentally wrote that he was 1 in my original post (typo) whereas he was 2, soon to be 3 in a few months.
In response to a few things, he had lost 50 grams in weight since the summer - we weighed him fortnightly. We didn't think it was a bad sign, since he put on 45g during the summer. I took him in from outside every day to check him and play, and the first sign of anything unusual was on Friday, but even then it wasn't much of a sign. He normally likes running about on the floor and investigating things, but that day he wasn't interested - except whilst that is slightly out of character, as I put it, it's not the first time he's wanted just to cuddle. On top of that, he was still eating fine at that point.
I wasn't there most of Saturday and I only got him out for a few minutes (not suspecting anything was wrong at that point), but again he seemed okay - eating, drinking, moving around. Sunday morning I came back and was told that he was being really lethargic and this was really the first sign of anything wrong. My parents had seen him in his cage not moving at all for a good 10-15 minutes and taken him inside. We kept him in all day because we were quite worried and got an appointment with the vets the next day, partly because we didn't think his condition was too bad (still eating / drinking / going to the toilet, but not moving much) and partly because our local vets are ridiculously expensive for a late night call and even more so for a weekend one (in the end the bill ran around £200). Later in the day, around mid afternoon, we tried to give him some lunch but he refused it. Thinking it was because he had simply had enough, we left him with the option of food and water in the indoor cage that we got from the garage. We noticed he hadn't gone for a pee in about an hour, though he had pooped, at around half twelve. Honestly, we probably should have gone there and then to a vets but I suppose we just thought that he was just behaving odd for one reason or another. He had started to move around again, though, which we took to be a good sign. That's probably what led us to let our guard down. At around one, he walked some circuits of his cage and made a strange noises so we let him out onto the floor to wonder which is what he normally likes doing. He did this for a bit and had started to seem like his normal self, so we were somewhat relieved. That's where we got the idea of his brother Monty stopping him from eating; presuming us giving him food had brought his energy levels back up. We put him back in his cage at two and it was around five o'clock when he stopped shuffling around, but that is the time that it gets dark around here and so it seemed like he was just naturally getting ready for sleep (or whatever guinea pigs do, because I don't think I've ever seen a pig sleep). Then that's when the noises started. He didn't sound like he was in pain at first, it sounded like the noise they make when they are angry at one another, but we got him out from his cage again to check him. He started burying his head in the crook of my arm at that point, which was a bit strange since he normally hates having his head covered. After around half an hour of that, I signed up on these forums and looked around the internet for some clue as to what may be wrong. Once I got accepted I posted on here and he started making the somewhat pained squeaks - that was when I started to get really worried. Shortly after we spoke to the vets and left. When we got back, we tried to help him relax and get him in the cage. After staying up for some time, I had to go off to bed. As I was walking away, he made a pained squeak so I went back and spoke to him. He had started shivering and I had no idea what could be done, so I just spoke to him. I doubted somehow that he would make it to the morning, so I said goodbye then. I tucked him in with some hay to stop him shivering (though I don't think it's because he was cold) and I left. After that there's not much more to say - he died at 4:29:33am today with us standing by him after my mum woke us up because she checked on him and heard labored breathing. He went fairly peacefully, though we still don't really know what caused it. According to the vets, he should have simply gotten better...

All of the guinea pig approved vets are 30+ miles away from us and we were already registered with another more local vets (4 miles) so we went there. Perhaps it was a combination of things that he simply couldn't handle? Anyway, sadly there is nothing that can be done now. I suppose we must simply remember the good times we've had with him
I was also wondering if you could help with what to do with our other guinea pig, Monty. He and Boris were like the best of friends and now Monty is all alone. Will he be okay? We'll give him plenty off attention
Boris, two days ago:
Boris and Monty:
Sorry if this post is really long and doesn't read very well, it's a bit late. Thank you again for your replies! I don't think I've been on such a friendly forum before
