Dear Parent, Thank you for your question. Sorry to hear your son has become lethargic over the past 3 weeks. How old is your son? If a child suddenly becomes more lethargic and tired over the course of a few weeks, this should be investigated at your family doctor. The most common cause is a viral infection. Children who get infections such as Glandular Fever can be lethargic for some time. Other viruses can also cause children to become more tired and lethargic. This would be indicated if he had a sore throat or a fever at the onset of the illness. There are other causes such as anaemia. Does your son have a varied diet with plenty of iron-rich vegetables? If not, this may be the problem. Does your son attend school - if so does he have any problems at school? Children who feel low or are depressed or stressed can often have a poor appetite and lethargy. There are other more rare causes of this problem which would become clear during the investigations. I would recommend that you visit your family doctor for him to have some blood tests. These should check his full blood count (to check for anaemia, infections and other blood conditions), as well as test for Glandular Fever and test his thyroid function. If his headache wakes him at night, is worst in the mornings or is occurring every day, a head MRI scan should also be included in the investigations. I hope you find this helpful, and the problem gets resolved soon. If you have any further questions do not hesitate to contact me. If you found this answer helpful, please leave a rating!