I wonder if they begin a craft with him, he gets up to play and an adult finishes it while sitting with the other kids because it can be incredibly therapeutic to color or make a craft. Just a crazy thought.
Seriously though, I know what you mean. My son is 33 months, and he comes home with a variety of projects. I try to pick out a few where he played a larger role and those are the ones that are displayed.
You could nicely mention it to them and let them know that you'd like to see what he can do on his own. He is still not yet 2 years old. Both of my boys weren't allowed crayons at that age, because they would just try to eat them. I did let them use washable markers. They didn't usually try to eat them, but they did make some lovely body art!
I just wouldn't be too surprised if they've tried and haven't had much success at getting him to do much. Give him a few more months and he'll try to do more.