I have a friend whose daughter (now in middle school) in 1st sounds like your nephew. In her sd, they test all kids for gifted placement. When she got the word that her dd had scored very high, she actually called the school to confirm it because her dd was having so much trouble finishing things in 1st grade. Her dd went to a gifted school for 2nd, and later was diagnosed with ADD (not ADHD). It might be worth a trip to the pediatrician to rule out ADD too.
By the way, this girl is doing great in school and is very much her own person.
