Hello ma'am. I can understand your concern. Poor concentration and increase spending time in phone games can be due to underlying mental health condition.
Stressor as per your history can be preparing for job interviews for 5 years.
However, a detailed assessment of your brother is required to rule out any other medical condition and get to know the cause of his behavioural change. Detailed assessment will also help to analyse proper treatment strategy.
Next Steps
I would like to suggest you to reach out to him and listen to him without being judgemental and also encourage him to seek professional help.