In a perfect world our children would be taught how to work in a group---waaay before they start getting graded on group projects. Good for your son that he is getting started early.
We had a technique that seemed to work well with our DI team. We had brainstorming rules. Every comment/idea was written down, as the kids were saying them---without any criticism or comment---Nothing was too out there. Often one "off the wall" comment lead to another which lead to another... Most of the best ideas were generated this way. Even if your child's idea isn't the one they ultimately choose, he will know that his idea was heard.
The minute a participant starts commenting or criticizing or judging the comments, the "flow" stops. So it's imperative that they understand this part the rules. They can set a time limit. We usually just let the ideas happen and cut it off when they dwindled down.
Once all the ideas are on the board, the kids can talk about the ones they think they can implement best. I remember kids saying "that's a really cool idea, but I don't know how we could do it"... It led to good discussions.
This is one of the hardest things for kids to manage. I hope the adult coaches don't let them handle it all by themselves.
