As you are a working mother, your son must have had some previous experience of being taken by you or someone else to preschool or a childcare program. Did he ever show this kind of behavior before? If not, there is probably something in the new program that upsets him. It doesn’t sound as though it is resistance to having his grandmother drive him there, as he gets upset when you pick him up in the afternoon, too. And if he is crying himself to sleep at night, something is going on that is really disturbing him.
Incidentally, a generation back this kind of behavior was very common during the first few weeks of kindergarten. Today, however, so many children have experienced daily separations when taken to childcare that they are cool and casual when heading for kindergarten. So I think there is something specific in his new setting and that you need to try to find out what it is.
Start by requesting an appointment with his teacher. This is important enough to take time off work; chances are, his teacher will want to see you near the end of the school day, anyway, so you shouldn’t have to miss too much work. When you meet, let the teacher do most of the talking. You may find out that the tears dry up within a few minutes of having his grandmother leave. If so, you’ve got a routine “school separation anxiety” on your hands, and it will clear itself up within a few weeks.
If the tears last all morning or all day, that is a different matter. In that case, try to get him to talk to you (or his father) about why he cries. At 5½ he should be able to give you a few important clues. If you learn nothing and the behavior continues unabated for as long as a month, I would try to find a child guidance facility in your area and have him evaluated.
Our parenting advice is given as suggestions only. We recommend you also consult your healthcare provider, and urge you to contact them immediately if your question is urgent or about a medical condition.