5 year old with severe separation anxiety

[Babylon5201]

My 5 year old son has always been sensitive and very close to me but recently he has developed really bad separation anxiety when it comes to going to school, he gets himself so worked up before school saying he will miss me his head and stomach hurts and even wanting to vomit, when it comes to leaving him at school he gets so upset saying he just wants me, he's scared and does not know why and that he worries something will happen to me. I've spoken with his teacher today as she is also concerned about how upset he is getting and see what we can do to make things better for him.
At bedtime he says he wants us to stay with him all the time and who is going to look after him, he has teddy's, nightlights but it dsnt seem to be enough, he does settle on his own after a few moments of reassuring him.
Does anybody here have any advice as it is heartbreaking as a parent to see my child so upset and at war with himself.