You have received some great advice about pictures of you on the desk, shirt, etc, or having an adult come get him out of the car, writiing/telling him a story, etc. I think these are all great suggestions. I do not think you have to resort to changing classrooms or pulling him out of school. He needs the consistency. If he's fine when he's at school, it sounds like it's a matter of really setting him up for success in the morning. At our school, parents do not normally escort their kids into the building. Kids are dropped off on the playground and escorted in with teachers. I think that helps a little bit with the separation anxiety.
What if, for example, you made up a rewards system where he would earn lunch with you (can you go in and eat lunch with him?) on Friday (or sooner if necessary) if he controlled his emotions better in the morning? Or perhaps you could dismiss him a few minutes early once a week for improving the mornig routine. Or, you could volunteer to bring in a snack for the class once a week. That way, his reward is time with you, but at an appropriate time and in an appropriate way.
Goodluck! (My son will be in K next year... should be interesting! I don't know if having me directly upstairs will be a good thing or a bad thing... but we'll see!)