Anxiety of an 8 year old

I have a son who is 8 years old and is struggling every day of going off to school, and leaving mom. He does go off with his dad to go fishing, to the store; he stay's overnight at his grandparents, for fun, periodically. He say's that it's easy because he is with family, but to go to school "he is leaving me" (his mom) and he will "miss me." Everyday it's a battle to get him to school and even for the teachers to get him to do his school work. He continually asks the teacher if he can "call his mom." He is in the 3rd grade this year and in the previous years of grade school, my husband and I have never had this problem with him. I don't know what to do or how to handle the everyday situation? I'm afraid he will eventually have a low self-esteem, feeling that his parents don't want him because they are kicking him out the door to school, and that's not the case. He has an older brother who is 10 and two younger siblings 7 and 2.