We went through the same thing with our old dog, Trapper. He was old, his hips gave out, he had tumors in his legs and his joints were seized up. He was such a lover, right up to the end, but he was in pain. We used a sling to help him up and down the 3 steps to our yard for a week or so, then we used it to help him up off the floor, then to support him while he ate, then we realised that this was no life for him, and was not how we wanted to remember him. We made the final vet appointment (our Vet is fabulous, by the way), and had a farewell party. We had a couple of days, and we wanted to help him out before he could not get out of his own poo. Everyone came by to say goodbye to him - which was a nice closure for all of us. We had a dinner party and gave him a people plate of people food, and he just seemed so relieved to be able to leave us and know we were okay. (maybe I am nuts to think I sensed this). As a final sign that we'd made the right choice, he did poo and could not get up to get away from it, or even to get outside.
While this sounds like it went on for months, the reality was that it was a few weeks from "Struggling with the stairs" to "Can't get out of the poo"... It's a tough choice to have to make for them, and if you are lucky, they give you a final sign that you've made the right choice - it was time, and you get to say good bye. Even though Spike is a year old now, we still look for Trapper when we come in the door...
Your family is in my thoughts as you go through this time with your sweet boy.