Hi, he may be suffering from attention deficit hyperactive disorder (adhd)or some short of learning disorder. In adhd generally sufferer have good memory but because of lack of concentration or high activity level he is unable to complete task properly. However this is a treatable one. Another possibility is he might be suffering from some learning disabilities. For all these please get evaluated from a psychiatrist for further management.

Kindly get him evaluated by a child psychologist for learning disorders. It is possible that he might be suffering from a reading disorder or writing disorder. He can benefit with special education.