D.K.
This is sooooo normal. It even happened last year with my daughter who was going into first grade.
It is normal for him to feel scared seeing other kids picked up, have the teacher show him a clock and watch as to what would be the latest point to even worry about.
I also did with my son was put a sticker on his shirt so if he felt scared or nervous I had kissed it and he had my kiss with him.
I would say try your hardest not to be late, easier said then done but plan your day so you are there on time. Being the last one left can really be scary and unsettling and he needs to feel like he can trust you and your mom. I do part time child care and I know that last one or even almost last one here at the house has a harder time. Plan your day however it has to happen to leave 10 minutes earlier,not much time if you think about it and be there for him when the other parents are.
You will build up his trust, lessen his anxiety and give him the reassurance he needs. If you do happen to be late, there needs to be that constant reassurance that you will be there and even having the teacher help comfort him that she has your number and will call.
Do you have a cellphone? Does the teacher have one with her? I know at my son's Pre K they carry one and if I was to be late I would call and talk to my son and tell him I was on my way.
He is internalizing his feelings and being worried so get him to open up about it. That is what is causing the stomach aches, have him tell you how he feels, reassure him and let him know it is okay to feel the way he does...
Good luck, it will pass after he does realize late or not he will not be forgotten.