ALL schools should have an anti-bully policy. Talk to the teacher, and to the principal if necessary. Don't encourage your DS to stand up for himself in any way other than to ask them to stop, otherwise he will end up in trouble himself. Definitely not what you are after. A good teacher will already have spotted what is going on and will be trying to deal with these bullies already. It generally isn't subtle emotional bullying at this age.

At least having had the staggered start your DS had made some good friends already and this should help him a bit if other kids start to give him a hard time. You might find that in the combined group with more nice kids around, the bullies won't be able to weild any power and will fall into line.