Talk with the teacher as he/she might be able to shed light on your son's school performance and behavior.  Perhaps he needs more challenging assignmnets to keep him interested and motivated.  Ask the teacher about your son's interactions with other children.  Do they play cooperatively, or is he teased or bullied?  That would be a valid reason for crying at a mention of going school...  Or maybe he does not want to go to school, because he does  not want to leave home thinking that something bad might happen when he is gone.  Don't forget to talk to your 5-year-old as well.  He is old enough to tell you what's going on and how he feels about it.

Best regards.