L.M.
Hello, I may not have the answer to why he is crying but let me tell you that you are NOT the only mother this happens to. I have 3 children 10, 8 and 4. All of which has had 4 years of Montessori schooling and what I found out early on is the seperation is hard. What they do in Montessori is they have you drop the child off at the door and someone ( a teacher) is there to greet them. That way the child does not try to control you and make you feel bad that you have to leave. My youngest did the samething, all of a sudden in the middle of the year she would cry and hang on me when I tried to leave. I would take her in and get her involved in a work or with a friend and then sneek out. It is usually a control thing with kids, their very smart creatures and they know how we as parents react to crying!!! Try telling him that you will stay for a while,(5 min ) and when he is settled then leave. I know this may not help much but it woked for me and always remember, he will be fine, it is always harder on us!!
Good Luck, L.