SJ the first thing I would do is set up a meeting with the teacher and find out how he is going and how often he is attending. You and DH need to do this and then perhaps the school will deal directly with you. You may want the principal to be present as they are the people who have the authority to contact social workers etc. If they know you are supportive then they will work with you to improve the situation.

Any absence from school is detrimental because they miss chunks of teaching and may not understand a particular maths concept etc because of this. It could also be affecting him socially because it makes him different from the other kids. It doesn't set up good patterns for high school and from my experience these are the kids that fall by the wayside once they move to the big school. If parents don't value education then usually the kids don't.

The princiapl could also talk to his mother and say if he doesn't attend school regularly then he would have to contact the authorities. My Deputy Principal would ring the homes of kids he knew would stay home for no particular reason or because their parents would let them and this resulted in a much better attendance (They always answered the phone). She doesn't need to know that it has come from the two of you.

HTH