M.P.
There doesn't have to have been anything that adults would recognize as being wrong or traumatic for your son. My philosophy is to follow my child's lead when they take me by surprise. I pause and think thru any possibility that they need me to provide firm structure or do they need open warmth. When they are this upset and it's out of the ordinary I do not force them to do what is causing the outburst. If the child doesn't use tears to manipulate then I assume there is something wrong, even tho it's not obvious. My goal is then to provide support for their emotions.
It's hard to tell what is wrong from your son's view point. I suggest you provide some quiet, warm, supportive time with him so that he's able to talk with you about this. Perhaps tell him a story about a little boy who was upset about being at school and ask him what he thinks the boy might be upset about.
I have kept both my daughter and my granddaughter home from school when they were fearful. They were ready to go back to school the next day. My sense is that they were in need of that extra attention from me and once their "love cup" was filled up again they were OK.