Poor thing. His world is a bit upside down really.
When are you moving back home do you know?
I would make an appointment to go in asap and try and get to the bottom of what is happening. I would say "right - we have had problems with the milk and the PE kit and now lunch, please tell me each and everything that we need to do to ensure my child is treated the same as the others as that is partly what is upsetting him".
I would imagine the behaviour towards his brother is him being angry about everything else and taking it out on the one person he is bigger and stronger and older than.
I have to say that with such a short time left of this term I would be tempted to take him out although then he will miss all the nice Xmas stuff that will be going on.