G.♣.
It is important to rule out any possible medical or psychological issues. I absolutely do not want dismiss that possibility. If he is otherwise healthy, I think you need to change your approach.
It is so natural for us, as mom's (and dad's) to want to help them, understand their feelings, empathize, etc. But it is usually better for them in the long run if you focus less on his feelings. Reasure him that everything will be fine and then act as tho,ugh it is. He will be much stronger if you do that.
When you walk him into school, be happy and excited, "Wow, this is going to be a great day! You are going to have so much fun!" If you know a couple of things he will be doing, you can talk about that. Is it music day, library day, art day, computer day? Maybe he likes PE? Whatever you can think of that he likes, go with it. Get really excited about it and talk to him about the fun he will have.
If you focus on trying to make him feel better or trying to talk him out of whatever sad or scared feeling he has, that usually just causes the child to focus even more on those feelings. It doesn't really help. You need to find a way to help him focus on happy and excited feelings.
Don't worry if he is sad or upset or even crying. Walk him to the door, give him a quick goodbye and walk away quickly. Don't stick around, don't wave to him, no prolonged goodbyes, even if you think you are reassuring him. You're not. You're just causing him to continue focusing on sadness. Turn him over to his teacher and leave as soon as possible. The teacher is in a better place to help transition him into whatever they are doing and helping him to forget the sadness.
This is honestly what he needs from you! You have to be strong. You can cry in the car after you drop him off, but you have to be strong for him.