Request a conference with the teacher as well. Ask her to work out a plan with you that will encourage your son to pay attention in class. The teacher could set up a system in which he earns checkmarks or stickers when he is paying attention and doing his work. She could send you a daily note to let you know how his day was and you could follow up at home with a reward of time alone with you or an extra bedtime story.
Since this school year is almost over in most parts of the country, write a letter to your son's principal describing the kind of teacher you would like for him to have next year. You may want to request someone who is warm and caring; firm, yet challenging.