Oh, bless you @Hugeroar (and @BetterWithPockets) and all those facing this most wretched of decisions. It's been explained well above.
I am a dog owner and used to work at an emergency vets - I just wanted to say that it's so, hard, so sad and so devastating. My heart is with you.
I have had five of my own dogs PTS over the years.
Please know that this an act of love that you are doing for him. You have loved him all this time and you love him still. That's why you are taking this hardest of steps at this most wretched of times. I well recognise the anguished soul-searching, the "is it the right time?", the sorrow and the doubt. I'm so sorry and I wish I could say something that would make it easier.
Your dog has trusted you all his life. He has relied on you for his food, his shelter, his medicine and his care, and his love. In all those things he put his complete trust in you. And he trusts you to do the right thing for him now. In taking this step, this final choice, you are repaying his trust. You are not betraying him, or letting him down. You are making the right choice. He isn't able to make this decision for himself - you are helping him to be free of his pain and are gently stroking away his future suffering. You are giving him the peace and the dignity he cannot secure for himself.
It is an act of love, what you are doing for him. The ultimate act of love, really. You are choosing what you know will likely bring you great pain and sorrow, in order to spare him from his.
Sadly I have seen the results of cases where there has not been such an understanding owner as you. Pets living long, long after they should have done because someone was unwilling or unable to end their torment. Not for your little friend this humiliating decline. You have amply repaid the trust he invested in you. He was right to love and believe in you. You have not let him down now, when it matters the most.
I am sure the vet will be kind. In all my years at the vets, there was never anything but dignity, kindness and compassion, both before and after the pet's passing. If you had an inkpad, you could maybe make some little pawprints on a card, and take a snipping of some fur as a keepsake (some private pet cremation services offer pawprint options as part of their package - I have used an excellent company called Dignity, specialising in private pet cremations). Perhaps afterwards, you could sow some forget-me-not seeds in a corner of the garden that he liked?
The countdown part is wretchedly awful. I'm sorry. Clear away his bowls and things as soon as you can afterwards, it is less painful than having to do it later. You can just put them away, out of sight for now. I know it sounds trite (sorry again) - with your decision will ultimately come peace, there will be smiles again, even with the tears. And the memories and love that you shared will be with you for always.
Please come back and let us know how you are after tomorrow, if you want to. Will be thinking of you.
Thank you for having the immense courage, compassion and decency to love him enough to make this cruellest of decisions. He is so lucky to have had you beside him, throughout his life, for now, and for what is to come. His great fortune in having had you in his life and the memories of the wonderful times you had together will not end with the passing of this evening. Those will live on in you, always. x ❤️💐