I think it's great that you're introducing him to preschool - in whatever means works best for you right now.
All kids adjust a little differently. Our kids have been in full-time day care since my son was 2, his sister was 9 weeks. At 2 and 4 now, they have completely different responses to adjustments in their schedules based upon their personalities.
Our son (4) takes several weeks to adjust to a new classroom/new teachers despite knowing almost all the kids in the new room. Our daughter (2) walks in like nothing changed from one day to the next.
I'd recommend dropping off and leaving. Don't linger, don't coddle. Just leave. The teachers are so experienced with working with kids upset with the transition.
When I was laid off last summer and bumped daycare back to 3 days/week, I always picked them up with some milk and a snack. We always ask the teachers how things were (so we can work on it at home) and have the kids tell us "all the great things" they did that day. It helps keep the experience positive.
If the transition is rough for him, we found that bringing in snacks to share, favorite toys to share (if allowed), favorite stories, etc. helped ease our son into things. Of course, we checked with day care to make sure it was OK, and avoided food allergies if possible.
A few weeks ago, we took a huge bag of fresh fruit (strawberries, blueberries, grapes). Sometimes, it's kettle corn from the farmer's market, GoGurt, etc.
Good luck. I hope he transitions well and you all enjoy the experience.