Your brother is lucky indeed to have such a caring big sister. Your dad may be right but it’s always a good idea to check out other treatable conditions before writing off someone’s behavior as a stage they are going through. Since your brother is complaining of being tired, the first stop should be his doctor. There are many medical conditions that cause fatigue. The thyroid is often the culprit. Sometimes the issue is a sleep disorder or a vitamin or mineral deficiency. If he hasn’t had a check up recently, your folks really should insist that he get one.
If he checks out medically, it’s possible that he is depressed. Depression doesn’t always present as being down. Sometimes the patient becomes exhausted and irritable. A common symptom is the loss of interest in things a person used to find pleasurable or exciting.
Finally, if he were seeing me, I’d ask him if he has become involved with drugs. It’s a difficult conversation to have but it’s still an important one. Weed, street drugs, and medications from the family medicine cabinet can cause a big drop in motivation and big changes in mood. If that’s the case, he needs help realizing what he is losing by using and he may need treatment.
Your brother may resist going to appointments. I hope you can help him understand that you want him to be checked out because you love him and don’t want him to miss the best part of being a teen.
I wish you well.