Awww! I am feeling your anxiety through the screen!
My ds has been in Montessori for 6 years- He just turned 8. This is the 2nd year for my dd. SHe is the same age as your ds- will be 4 in May.
The primary classroom is so appealing to children, so many things right there for them to touch, examine! And because of the community feeling the teachers provide, they will definately make him feel right at home, as will the other kids.
How long will he be there today? I would say "enjoy your quiet time" but I know that will be impossible for you today! Maybe tomorrow, though!
He's gonna be so excited to tell you all about his new school!