You can help your son learn to do frequent checks to see if he is daydreaming and refocus his attention. You might also make an index card-sized note or picture (a smiley face, perhaps) to tape to his desk. When your son sees the card, it will remind him to pay attention.
Talk also with the school counselor. He may be able to observe your son in the classroom and give you an objective opinion of what is going on with him there. The counselor may also have some behavior checklists that you and the teacher could complete to give everyone a better idea of his attention problems.