J.E.
Maybe you could set up a conference with the teachers and your son and you could all talk about it together. Maybe hearing it reinforced in front of you that he can talk to the teachers will help. I wonder if the teachers could have the kids do some kind of activity where they have to work together. I would ask the teachers exactly what they would do if he were to approach them with a problem about the boys so that he would know what to expect. It seems like the more information children have about the specifics of what's going to happen, the better they respond. I wonder how much attention these teachers are paying to what's going on if they aren't noticing this. How many kids to a class?