I don't think there's anything *missing*, per se. I just wonder what happens when he cries with your mom. You say she's calling you... I think both of you can work on a plan to let him know he IS fine and right where he needs to be.
If he's missing you, I like Diane's suggestion of a little treasure. I had a preschooler in my group who had a pattern of having hard transitions; he used to have a few pennies in his pocket that he would hold onto, and that seemed to help him.
I also acknowledged the child's sadness at the separation, but then handed it back to them as to how they wanted to handle it. If they wanted to sit alone in the cozy corner or another quiet place and cry or look at books or snuggle with something, that was fine. I also offered : "Let me know if you want to write momma or daddy a note". This, I found, has worked really well with a lot of kids-- they get their feelings out, we put the note in an envelope/in their bag, and then they usually are ready to be in the present space with the other kids and caregivers.
That said, when they were *wanting* to sit and be sad and feel sad and wallow in it (and I do not say this unkindly-- some kids do have this temperament), I would just tell them that "it's okay to feel sad-- come join us when you are ready to play". This way, we acknowledged the feelings and that they were okay, but things didn't come to standstill because of the tears and drama. This isn't a new transition for your son, so staying very grounded and centered around it will help him. If WE treat it more like the end of the world, it will amplify for him as well.
Good luck! (Oh, and don't forget the Kissing Hand book by Aubrey Penn. That's another nice tradition some parents use!)