Sorry to hear your anguish over this but what what he is experiencing is normal even if it appears that all of his classmates are sitting and involved in the activities. Many children at this age have a hard time at drop off but then do quite well during the day. If he wasn't doing well, then the teachers would express their concern. Leaving them at school is often more difficult for us as the caregivers but usually after a few minutes, they do just fine. He is attached to you and the rest of his family so it is natural that he desires to be with you, but most likely he is doing just fine after a bit. And you can always check in with his teacher to ask about how he does after you have left and if he is involved with the activities.
I am here when you are available.