Your son's diet is excellent with the possible exception of his breakfast, but then he is not too keen on eating.
I think that you already know the reason why he is so difficult lately. Just look at your first sentence again where you say that it all started since he has been at school this year. He is seriously unhappy at this school, so much so that he has lost a lot of weight and his behaviour has become a problem. Being the smallest and youngest in his class may have opened him up to being bullied. You need to ask him and his teacher to find out if there is any obvious problem. Again I strongly suspect that he is being bullied and there may be more than one of the boys in his class or at the school that are doing this. He needs to make at least one good friend at the school. Invite this friend to your home. If you cannot solve the problem easily it may be necessary to ask if he can change to another class. He needs lots of understanding and support and his dad needs to play a role in trying to boost his confidence.
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