My son was like that two in fact he still is till about 9 am the things that worked for him is when he got frustrated they would walk him up and down the hall then come back to it. Also when he didn't finish or decided not to do it when he got home we did do it he wasn't allowed to play until he did, I know that sound cruel and believe me he made sure to make life miserable for the first three days but when he finally understood if he he doesn't do it at school he has to at home he grudgingly started doing it at school.

Please make sure rewards are being presented to him and high five praises when he do complete task every time also ask the teacher to find something that grabs his attention that's fun to get him though his task through out his day. I found out with our children we have to make things fun to grab their attention to get the job done. Like for one my son his favorite subject is sciences and he love puzzles and his tablet in between these things I used these activities to get him to do a task at home if its helping mom clean up some or clean his room up. It works for me now he gets to pick a choice what he wants and the same for a snack I do the same. When you rewards our children its a big thing to them and it helps with the behaviors that occur.

What ChristineL said about biomed
Also is he on sugars and processed stuff?
Have you looked into gaps?

Good non processed diet and Pycnogenol is supposed to help a lot for adhd