Final answer:
The most likely diagnosis is attention deficit hyperactivity disorder (ADHD) due to the symptoms of inattention and poor grades. ADHD is a neurodevelopmental disorder that affects attention, impulse control, and behavior. The boy should undergo a comprehensive assessment for confirmation and treatment planning.
Step-by-step explanation:
The most likely diagnosis for the 13-year-old boy with inattention and poor grades is attention deficit hyperactivity disorder (ADHD). ADHD is a neurodevelopmental disorder that affects a person's ability to maintain attention, control impulses, and regulate behavior. The symptoms mentioned, such as difficulty following instructions and avoiding reading/writing, are common manifestations of ADHD.
ADHD often co-occurs with other learning disabilities, such as dyslexia and dysgraphia. It's important for the boy to undergo a comprehensive assessment by a qualified healthcare professional to confirm the diagnosis and develop an appropriate treatment plan.