Sorry about what you are going through with your dog.
Happy Birthday to Izzy, and it warms my heart to see older horses that haven't been discarded but are being well taken care of and living with their horse family and human friends.
I just want to say I believe you are that kind of person, and so I know your issues with the dog have not come about easily and you are only struggling with dealing with the dog because it is exhausting and you have not been able to find answers to help him.
In my mind, it is similar to having a mentally ill older relative and wishing you could ask them to live with you, but knowing it would be impossible, so they have to go into a facility (personal experience). For humans, there is less of a way out for suffering, but we can help animals more easily sometimes.