For a 5.8 year old to 'rule the roost' so much that he gets to say that he is not ready for breakfast an hour and a half after he has woken up is... not great.
Get him up, expect him to get dressed, washed and ready for school, then serve breakfast and sit down with him to eat it at, say, 7.15. No playing beforehand, no tv, no attention, just 'it's breakfast time now, come and sit at the table'.
He sits at the table either until he has eaten a decent breakfast or you have to leave, whichever is earlier. Reward the desired behaviour - if he eats. Ignore the unwanted behaviour - if he chooses not to eat. Express your disappointment with him that he is choosing not to learn his best at school because he is choosing not to eat if he has not donw so before you leave the house, then don't mention it again as that period of poor behaviour is over until the next day. Arrange with school for your teacher to reinforce this message if he's still not eating breakfast after the first week.
It is your job to use behaviour management at home to ensure that he eats breakfast. There is no issue for the school to deal with, because their policy is set on the basis that a child will arrive at school having eaten breakfast.
My reply to you would only be different if you were genuinely not in a position to give your child breakfast - at school we regularly give breakfast to children from families too chaotic [as in with alcohol / drugs / mental illness / domestic violence issues] or without the money to give their children breakfast.