Cathy, let's try to remind him the things he enjoys at school, e.g. food/snack, games, toys and art work/craft. You may also encourage him to voice out his feeling e.g. I understand that you've not adapted to your school life but remember Mami loves you always and will always take you home after school.
Don't underestimate our little ones. They understand a lot of things. Their crys may be due to their fear in facing new environment or because they cannot express themselves.