Can a older horse just get to be too lazy to eat?

My own 22 year old hard keeper Arabian gelding had about the same amount of Purina senior daily, gr hay 24/7 in the run in shelter, rice bran for extra fat, and 24/7 pasture which he spent much of his time on. This kept him in good condition. But when he wanted to get in out of the sun or away from the bugs, there was always hay in front of the fans for him to chomp on. My point though, is that he was never too lazy to eat, but he did like to get into the shelter periodically. If you can provide fans and hay in a shelter for him to access at will, he might do better. also be sure he has plenty of fresh water and a salt block available at all times.