perhaps, he is bored with the pen and paper worksheets. Afterall, he gets that from his school teachers.
Y dont't you search some online educational lessons/games for him. There are plenty of those.
Or you check some printable online worksheets. Show it to him and let him choose from your selection which he prefers to do. That could get him excited coz he is part of the worksheet 'setter'.
Another method that most probably works, tell him that if he finishes his work, he will get his outdoor playtime session. Eg: Bring him to the playground, let him cycle outdoor, etc.
Time to study and to play...that can never go wrong.