I do not have any advice for you. But I just had to put my 27 yr old TB gelding down on the 26th of June. He had cushing's disease and normally during the winters the TB's would lose weight but bounce back in the springtime. Our other TB did, he did not. His coat was already dull and he didn't seem too interested in much of anything, ie, eating, drinking, going out of his stall etc. so finally the day came when he kept going down and having a hard time getting back up. The vet came and said, although it was our decision on whether we wanted to prolong his life longer, it was in his best interest to be put down, she also said she believed he had in fact developed cancer in his intestines and that it had been slowly spreading. We had him euthanized that very day. Although it was extremely hard on us, we just couldn't let him suffer.
I believe if he isn't the way he used to be, then maybe it is time to let go. Horses body language and just their general overall looks change when they aren't feeling right or when they are trying to tell you it's their time. I think maybe he is trying to tell you he is ready. You are just going to have to put on a brave face and tell him "it's okay, you understand he is in pain, and that you love him." I would have the vet out and have him humanely euthanized. In the end it is a VERY hard decision, but, do you think he deserves to suffer because you can't let go? Or that he deserves some peace. Good luck and I am very sorry this has happened.
Guy 36 yr. old TB gelding, Cash 26 yr. old paint gelding, Kodie 7 yr. old AMHA/QH gelding, Hillary 6 yr. old X mare. R.I. P. Street Chief...1/5/85-6/26/11 TB gelding. Hating this hot and humid weather!!!