Sorry to hear about your dog. That is is tough situation. All the previous posts are good advice.
We put our 15.5 year old shepherd down last year. She had DM and could no longer control her bowels. When we had to pick her back end up to get her up and outside we knew it was time, She was fine in the head but she knew it was time too.
Its super hard to do, the hardest, but be strong when the time comes, which may be very soon. When you know your dog isn't going to get better, then its time. It gotta be all about dog, not the people.
Plan it with your vet now so you are ready when you need to be.
Just be there for your dog as he passes, it will be ok.