The reason seems very clear. He wants you to stay with him longer which is actually not a bad sign.
He just dont know how to express and u of course might not realised.
Solution need to asseess his ability and interest.
Saying like if he is clever enough, u might make him an offer what u could do when u are back home if he behave well in morning.
But ever never never bought him a snack or too materialistic reward.
I am 100% sure, the case will get worse later if u did it this way.
Choice could be a play in playground, go to Mac or even watch a video if he behave gd enough.
it should be an event, a function with some interaction at least.
One more crucial thing is that even he himself forgot what u promise, u should do whatever u said before at night, even reminded him. It is very important.
Other tactics could be written later if u want to know.
or u might write more details for each part if u want to know.
My daughter (3 years old) has similar problems, because it is her father brings her to school. When she wakes up, she cannot see me because I go to work very earily in the morning. She will ask where is mother again and again even my husband told her she goes to work and will pick her up after school. She is not like that before and just happens from this week. She studies at this school for about one month and before didn't have problems. I just tell her at night that I would pick her up in the afternoon, She is not good at interpreting questions and only knew how to speak two to three components sentences. Would anyone share their experience? Thank you!