Oh Callum
Have you taken him to the vets yet? Although it doesn't sound good
I had a dog once whose legs went like that and then he just got worse
If they are suffering in any way, it is much kinder to let them go, however much it hurts to do so, rather than them suffer
I had to have my rabbit put to sleep last October, I cried for a long time
But knew I was doing the right thing, I made his last day as happy as I could, made him warm, comfortable, plenty of cuddles, food he liked and could just about still eat, then took him to the vets, and kissed him goodbye. He was eight and a half years old.
Yes, I echo what Bobnot says about them having a good life. It feels a little less harsh to let them go then, as you know they were well looked after and had a happy life.