Thanks everyone.
It's Friday morning, and it's not good. I slept downstairs with him last night. This morning, he was able to get himself up to go outside to pee, but he didn't have a bm. He is still having trouble with his hind quarters, and his breathing is more laboured than it has been in the last while. He wouldn't eat a treat, and I had to hand feed him his breakfast. Even then, he only took about four mouthfuls of chicken, rice and canned/kibble.
We have decided to nurse him through today (I'm off work today); let the kids say goodbye to him tonight, and take him to the vet tomorrow for the final time.
Even as I type that, I can't believe it. I know it's the right thing to do but man, it sucks. I just want him to keep what dignity he does have in place, and like you all have said, I can see by looking in his eyes, that although his spirit is still strong, his body is telling us all it's time to say goodbye.