kids excluding someone

I teach a farm school class of 5 kids. There are 2 girls who are 8, and 3 boys who are 6 years old.
4 members of the group are choosing to band together and to leave out the youngest boy. From the mother's reports, this has happened in other contexts as well.

How do you encourage a child who is being left out to grow into a person that others want to include?

How do you encourage children who have determined that they want to separate from a single child to be kind and to find ways to connect with the "odd man out?"

Thanks