It breaks my heart to read this, my Charlie will be 13y/o in April so far so good he is sleeping a bit more but otherwise doing good for his age. I lost my ESS 2 years ago at 12y/o she started going senile wandering about and toilet in the house also loosing perception of where she was. We made our decision when after a vary bad urinary infection constanly leaked urine no amount of anti biotics seemed to help. Then when her abdomen looked to be very swollen the vet thought she may have a tumour, we decided not to let them take it any further and said a tearfull goodbye to our girl, she was ready to go I saw the look and would have loved to ignore it but knew she deserved better.
Hugs to you both I hope the vet can give Charlie something to help I so know where you are just now, they are never with us long enough I so dread the day when my Charlie starts the downward decline like you I will do whatever I can to keep him healthy and have a good quality of life.
Cathy