D., this is a sad time...
I'm thinking, if a sticker helped a little (so cute, to kiss a sticker, you're a great mom!!!)
then maybe a cuddly cozy favorite teddy-bear or other stuffed animal could go with him.
Maybe you could tell hm that the teddy-bear is missing him at homne,
and he also wants to see the world (the bear, i mean)<
and wouldn't it be nice, if your son were so nice and took the bear out for a couple hours walk and by the way take him to the daycare also, to show him (the bear ) the world a little> And, before you go, hug the bear dearly and whisper into his (bear's) ear some good words and then tell your son that he will also look after his best friend (your son), for you, as if passing your love over to him every single second when you are not together.
You know, to make your son proud and responsible for caring for his bear, and an object from home with him, to feel cozier also. It sounds rather like the friendship of Winnie the Pooh and his best friend Cristopher Robin... :)
maybe it'd help your son to switch the attention from feeling lonely and abandoned by the beloved mom, but instead, to start feeling confident and important at the time of being trusted with caring after another soul (oh well, the toy soul, but this is how we all start, right). It might leave him no time to be sad or frustrated.
Especially, if you talk to the daycare teacher and make sure she will support the 'play'. for instance feeding the teddy-bead also, and inviting the bear into the circle of listening to the story and play.
It is all good to try out IN CASE if it is really all right in the daycare. I do not know how to check on the place and people there, but with this abrupt change, it really seems like something scared your boy there...
Not meaning to say anything bad, but life is life and this world is pretty harsh. I know a girl whose real FATHER molested her from when she was 3,5 , but she spoke out loud about it only when she was 16 (!!!) they lived in one house until she was 9, and it all went on, without mom or anyone knowing the situation. There were some strange lines of behavior, but nobody could actually put it together... once she spoke up. all the puzzle pieces finally fit together. Now think about it, why she was quiet for SO long, and never asked for any help??? Part of it is: scared. Bad adults know how to make kids be quiet, by scaring them: if you tell mom, ... oh, anything, up to the point that she will die... I've heard stories, but THIS girl I know very closely.
Another thing: maybe, if you ask your son, would it be okay for him to try out a DIFFERENT daycare: what would his reaction be? If he will feel extremely happy about this option, then you may be more cautious about this current daycare, then there might be some hidden reason. If your boy will be as reluctant with a new place, then maybe it is not about people in the old place...
I don't know more, but this is all I could think of, D.,
and with all my heart, I wish all your family the very happy days! Take care!!!
M.