You are asked to see a 5-year-old with a history of headaches every day for the last 3 weeks or so. The headache always happens in the morning and he has missed a lot of school. His mum is worried because he has started complaining of seeing double in the last few days. He also has been vomiting. The past few days she has noted he is a bit off balance sometimes. There has been no recent illnesses. No fever, no diarrhoea and no other pain anywhere else. He has not travelled recently. There has been no rash or other skin changes. No seizures.Your examination reveals no weakness or sensation abnormalities. His pupils are equal and reactive. His cranial nerves are intact. There is mild ataxia. No rash or meningism. The rest of his physical examination is normal.What is the most likely diagnosis?