My 5 year old boy cries when i leave him at school!?

He used to do it at preschool a lot, then when he started school in september he stopped doing it! Then for some reason it started again, i just pretty much ignored it, said goodbye to him as normal and left him with his teacher, who told me once i'd gone he settled down! Eventually this stopped. But every thursday morning the school run a session called stay and play, where parents can stop for 30 minutes and play with their children in the classroom, i enjoy doing this but my little boy just cries and fusses the whole time i'm there because he doesn't want me to leave! He's also started to cry again at home when he doesn't get his own way (he's generally a very good boy) which is not tolerated. I just dont want to end up with a whingy crying little boy and am confused as to what to do to stop it! I have tried talking to him!