Pretty sure it's not to do with training. A similar thing happens to my players sometimes. I find a great way to boost morale is to have a moan at them about their training.
Just say, you are going to have to work harder to stay in the first team!
95% of the time, they admit they should work harder, and they get a morale boost. I do this to every player every month. And litereally every single one of my players has me in their favoured personnel. Even ones I never play.
I used to, but then I realised it makes no difference either way.
They could be training very hard, but they'd still admit to not training hard enough. And they never actually train any harder after moaning at them. I've even tested it by reloading the game when they disagree and say they have been training hard. It's basically random as to whether or not they react well, but the vast majority of the time they do, and if they don't you can just say Fair Enough and then they'll still be happy that you were understanding.
Compliment him after good performances (above 8 match rating) and if he plays bad (6.3 or lower) tell him so. Players like it a lot, when you show you're interested in them. You'll get 1-2 morale levels worth of boost. And try what Sheva Elite said, it has worked for me as well.