Do you have low jumps/poles to go over for fun? Horses like variety. They easily get sour if they are worked and doing the same thing everyday. Yeah just groom him, whisper sweet nothings to him, and give him treats, he'll love that. Maybe he feels like all people want from him is WORK since he hasn't been with his "mom" for a while and doesn't get the same one-on-one attention that she gave him. They know the difference.