Sometimes if the class is large kids can really get lost without many noticing. In a smaller class it's easier for teachers and admin to see these things and help. I suggest always talking with the teacher. They should be there to help. They have a first hand perspective and if they are any good should at minimum help foster an environment that will help him feel included. I think we've all been in your shoes as parents, it's not uncommon. At minimum he shouldn't be eating alone. Teachers should recognize that.