Have you talked with the school about how they help you transition? It sounds like you are doing great starting to prepare him now. Make sure you are explicit about letting him know you will not be staying at the school, otherwise he may just be thinking it will be a new place to go with you and he won't be wrapping his head around going alone. Talk about how other children will be there without their parents, how there will be a teacher to take care of him, etc.
The most important time is just before he starts and when he starts. The week before, drive by the school, go in if you can, show him where he'll be going, talk to him about the fun things he'll be doing.
I have found whenever I leave DD at something she has a great time, but the actual leaving is the hardest. A lot of daycares and preschools try to "help" by picking the child up and carrying them off and trying to engage them, but this is often traumatic for the child and it irks me when they do that. For my DD, the best transition is if I come in and stay with her while she adjusts to the environment. Often, she will take 5 or 10 minutes and just look around and then get excited about what the other kids are doing and will join them. I can leave at this point without incident (I do make sure to say goodbye either before she gets engaged or after). However, if the staff tries to engage her the moment she walks through the door, it will be a rough transition.
So, its useful to talk to the staff about how they handle it and how you would like to have it handled. Make sure to let them know, you don't mind staying for 10-15 minutes to get him settled. I think a lot of them assume you are trying to get to work and need to leave immediately.
Remember too that he may say he doesn't want to go when he really does. My daughter went to a little preschool / creative arts thing two days a week from Feb to June this year. She loved it, she was excited about it, if I told her she was going tomorrow she'd dance around, she'd be all excited all morning. Recently when we went over the summer to the same place, she'd be really excited then we'd get there and she'd say 'I don't want to go!' This was frustrating me to no end! I found that if when she said 'I don't want to go' I said, okay, and we shut the car doors and looked like we made to leave she'd immediately say 'I want to go!'. So just keep in mind what they are expressing verbally may not match what they really want. She probably just doesn't know how to tell me what she really wants.