I have had the pleasure of having 4 senior horses in my life, 2 of which are still with us and 2 that have passed on.
My old guy Elmo , who was also a resuce, would go into deep sleep like you say your guys does. He was deaf as a post and if my old mare Cheyeene did not nudge him awake when it was time to feed then we would have to go out there and shake him awake.
We knew that it was getting close to his time when we noticed that he was sleeping more and more ... and he started to have a hard time getting up, he started to seem less than thrilled about his feed and would walk away from dinner (the vet did check him and everything medically was fine).
Then one day we came out and he was down and just was not able to get back up, he had REALLY bad arthritis in his front legs and a very old injury to his back leg that was never treated and just left to heal so he was gimpy. We decided that he had been thru enough in his life and that it was time to let him go, so we made the call.
Now as for the dominant to mellow change that might just be due to age... my daughters mare, who is now 36 years old, was the QUEEN B in the pasture no matter how many horses or ho big they where, funny to see a 14.1 hand mare boss a 17 hard Percheron around, but over the past few years she has just mellowed out. I think that she is just to old to care about who is in charge anymore, she will still stand up for herself and get her point across loud and clear if she needs to
Good luck with your guy and glad that he found a home with you