Take a few steps back. Each day, instead of focusing on the notes, have some downtime prepared. Pack a snack bag and head to the park so he can get some wiggles out. Or have some coloring books and a new box of crayons just for the house on the table to work on. Or set up some of his cars and have a road rally.
Don't talk about school at all. Just love on him evne though he was rotten at school. When you get back home or are done w/your activity, go through your evening routine and then the next downtime, usually at bedtime, ask him about school. But instead of saying "let's talka bout this note", ask him "did Johnny do anything funny today?" or "Did anyone talk to Sally today?". That'll get him focused on what was going on in his class but still not focused on him. Keep him talking and see if he mentions any incident that may have tripped him up, or if you see patterns in his day that trigger emotions.
Or, he could just not like so much structure, direction or even the teacher!
I guess this is a roundabout way of askign directly how his day went and what's bothering him. hth and gl!