Erm, well, only the school knows if he is alright or not. But he might well be fine especially if they are telling you he is.
I vividly remember my first days at school and without wanting to project my own situation onto your child, I was not fine at all, I was really, really unhappy...but after a few days I began to cope.
I coped for the rest of the time I went to school but was never happy, therefore I think if it were my child, I'd want to consider if they are just upset on a shallow level or if there's something deeper and more 'genuine' going on. With ds1 it was something deeper, but he took at least half an hour to be able to function, clinging on to the assistant in tears, and he was never really happy, though this was preschool. (he was nearly/just four at the time and I was very pregnant).
I took him out and we stayed at home with his new brother. When september came I was afraid he would be as upset starting school but he wasn't, he was ready by then. I think it was other stuff going on in his life that made him so unable to cope for a while, and once that was dealt with he was Ok.
Just don't underestimate the feelings they can have.
FWIW if you DO feel he isn't ready yet, you can defer entry till January or Easter for him and the school is obliged to keep his place.
(we are doing this with ds2 whose birthday was in June)