An 8-year-old boy is brought to the clinic by his mother with complaints of difficulty concentrating in school, frequent fidgeting, and inability to sit still during class. His teacher reports that he often interrupts other children, blurts out answers without raising his hand, and struggles to follow multi-step instructions. His parents note similar behaviours at home—he loses his belongings frequently, forgets chores, and has difficulty organizing tasks. There is no history of seizures, head injury, or developmental delay. Hearing and vision are normal. On examination, the child is restless, makes poor eye contact, and shows difficulty sustaining attention during the assessment. What is the most likely diagnosis?