Hi L.,
I could just cry reading your post- such a hard time (for your son AND you). Great advice already, just wanted to share my experience for what it's worth. My daughter was also a clingy one (still is- 4th grade now).
I did find it helpful to ask the teacher how she handles these kind of children. They have been through many kids who have a hard time leaving mom, and are usually nice.
A quick exit does work best. If you keep giving 'one more hug' and keep talking to him over and over, it just drags it out and really does make it harder. I found with my daughter that saying ahead of time exactly what would happen made it easier. "I will line up with you and hold your hand up to the door, then give you one kiss and hug and then you will go in". This made the morning predictable for her. She often still cried, but she was just sad, not freaked and floundering, and therefore not anxious. Read lots of books about starting school, come up with your 'plan' for each morning, and even practice it.
Your plan sounds like it will involve the bus, which might go either way. I do think if he's excited about that, let him do it. But make sure he knows what it's all about. If you have time, head on down to Douglas County where school has already started and let him actually see what it's like to get on the bus (if he hasn't seen it before). Then, with him, come up with the plan to see him off.
The best thing I did with my daughter was to have the practice, stick to a plan, and NEVER make her feel bad for how she is feeling or acting. Suggesting that it's been long enough or whatever is simply pressure they don't need. Best of luck!