Please help me understand my kindergartner ?

I have a 5 year old this is his first time going to a big school he did go to pre school and he did fine I am guessing because it was a half a day of school he now goes a full day of school every night he tells me mom I cry at school because I miss you all day I tell him well best buddy I miss you to while your at school but when you get home we are together again right well today while aboarding the bus he refussed to get on the bus I told him he will be fine that I loved him and when he gets home his package his grandpa is sending him will be here I was trying hard to make him feel good I need help has anyone ever went through this and what did you do to help your child over come his/her fears of missing you i need help I feel bad for him