Is he anxious about starting 1st grade at a new school? I'm a teacher, and I gotta say that 95% of kids who are anxious are okay after saying goodbye to their parent at the door. They stop crying and cope just fine. If he's only anxious about the bus, then drive them to school, but drop them off at their classroom, or line-up, or wherever. When you meet the teacher the day before, ask her/him to review the routines and procedures and then re-inforce them the evening before (e.g. If I need to go to the bathroom, I...) with your DS. Also ask about first day procedures, so DS knows where to go.
I suppose I'm biased, because I'd rather start the routine off without parents in the classroom, but a caring teacher will make sure that little ones feel secure at school, by being very clear about routines and expectations and timelines. Anxious parents can unintentionally interfere with the child's need to learn routines and start to form peer relationships on their own.
Talk to the teacher the day before and see how DS feels. Chances are he'll be just fine!