I had a talented horse years ago. He'd get bored. He would get sick of the same old routine. We'd been working on changes, mainly on waiting to be asked for the change and he was sick of the structure. So, he was sullen and cranky. I didn't do any dressage with him all week. We rode down the beach. We went to the roping arena. He chased steers out of the box, ran them back down the alley...had a blast. My trainer asked what we did during the week he seemed pretty keen. I said, "chased cows" waiting for the shoe to drop. She said, "Keep chasing cows, it's good for him." Find something your horse loves. Once you determine it isn't a health issue or injury that is.
Vary the routine as much as possible! Cross training and trail rides are my favorite.
Try to break the everyday routine, get him out of sand arenas as much as you can, take him to the paddock, take some time to look after him and care for him (treats, massages…). When working always keep calm and reward him when he does what you ask him well. Give him some rest when he needs to during show season. It is a generally peaceful environment, a good health and a good diversity of work that will keep him happy :)
I advise you to ask the veterinarian to do a blood test to see how the health of your horse evolves.
If he has changed feed, this can also play :)
If you know the cause of the decline in morale, you can change a little your habits (come see him without necessarily ride for example). At work, you can reduce your demands, intensity of work but also the exercises.
Finally, going out to the meadow with his friends can be a good solution :)