This may not be what you’re looking for, but here goes.
“How can we motivate the kids to care”
Unfortunately, you can’t. This is not in your domain or a controllable position for you as a parent, not a coach. In fact, there is a coach(s) and that’s their job, not yours.
Also what’s unfortunate, is that this is one of the points along a learning curve of life. I know you’ve probably experienced something similar at work and even within your own social circles.
Now trying to explain all that to a youngster is probably like taking to a brick. But, this is your son, not a brick. So look at it this way, it’s an opportunity to show your son a little of life that’ll help him understand the bigger picture tomorrow and all the other tomorrows that follow. In that regard, it’s all about presentation and how things are worded.
First, give your son credit for seeing what you see. Compliment him on his ability to observe and make sound and rational judgments of what he sees and what he’s going through right now. Also, compliment him on sticking to it, playing through a difficult time, and not giving in to what everybody else seems to be doing. State the obvious – he as a young man, gave his word that he’d be there, ready to play his best regardless, dependable and worthy of pulling his fair share. Then tell him that you’re so proud of him for doing things that many adults that you see every day, don’t do.
Your son deserves a lot of credit for backing up what he said he’d do at the beginning of the season – that’s what a man does, his word is his bond.