I remember one child in particular who cried *a lot* when he started back when I was a preschool teacher.
I sat with him on my lap for pretty much the *entire* first day...and most of the 2nd. (as I remember we also had a sub teacher that first day so that was even more interesting...)
That's what I did. I talked to him about whatever the group was doing while he sat with me...gave him opportunities to try things out, but never forced it. Everything was "that's OK" pretty much.
I took him around the room at free-play time and showed him our toys.
After 2 days, as I recall, the crying went from most of the day to only right away and maybe at snacktime until he was totally reassured that he did *not* "have to" eat. He started to play with our toy school buses. And every day for a long time after that, he played with the toy buses. It took him awhile to join in our snack too.
But he DID adjust, it *did* get better. What I would do in this situation is stay for a set period of time if you can, or think it would help rather than make things worse. (some kids it's better, others it's better if you leave.) My own son I stayed his first day for quite awhile, I'd say an hour or so, and he was fine after that. The next day, I left, and he cried when I left, but only for a couple minutes. Then he was OK...and now he loves school. And I think for him, staying would've prolonged it and made things worse. Only you know the answer to that one.
I think it is better to try it for a little while, whether you stay or not. It takes children a few days to understand the routine of the preschool classroom, and once they realize there is a routine and understand what comes next, they get a sense of stability that I think helps improve the separation anxiety. (They understand that XYZ that they like happens every day at this time, that they truly will not be *forced* to do anything they don't want to do, they have gotten to know the people a little bit, etc.)
If, after a few days, the crying has not improved *at all*....that might be the time to think about him being 'too young' or 'not ready yet."
Also...I think that NOT giving the child the chance to adjust to it sends a message that you think they *can't* handle it...what I think those first days do for a lot of kids is....they see that it was unfamiliar and scary at first, but then they got to know the people, the routine, see the toys....and they learned to *enjoy* it.
What a major boost...the realization that they *can* come through a situation that was scary at first, but now it's OK and they *like* it.
I think there's potential for a future situation to go *worse* if a child has a "failed" attempt to look back on...wow, school was scary...and Mom took me home...I *can't* do this...
Nobody wants their child to cry, I understand that. Nobody wants to force a child to grow up too soon, or stay in a situation that they're not ready for. I get that. But....they also need to be given a small chance to spread their wings and see if they can take off.
If you are in a school where the teachers truly care about the children and are present and willing to help a child who has a hard time adjusting to being away, I think it's fine to give it a shot. Your child will *not* be sad alone...he will learn to trust that these people will help him if he needs it while he is with them.
Oh! And the other thing we did with that little boy that I remember is, we had a schedule up on the wall, in pictures, of our day. We looked at that *a lot* while we talked about the routine and I was able to show him when he would ride the bus back home. In a way that a child can easily understand--snack followed by play, then circle, then bus. Over and over and over till he understood when he would see Mom again. This might work for your son too. (mom can stay till X, then you will do Y and Z and then it will be time for me to come back!)