I wonder if your DS is picking up on the distraction/enthusing thing and it is having a reverse effect?
I know for dogs (which, let's face it, aren't all that different to humans in lots of ways) if the issue is barking because they are nervous, the recommended response is not to try and get their attention/distract etc, because what it does is reinforce to the dog that there really is something to worry about!
Perhaps it is similar in this situation? You are anxious to get your DS over the 'dread of school' thing, so you provide more and more jollying, distraction, enthusing, so your DS starts to think 'flip it really is that bad!'
It might be helpful to just wait until your DS mentions that he doesn't want to go and simply say 'I know, it's horrible when you don't feel like going somewhere, but I'm your Mum and it's my job to make sure you get to school on time. If I don't, I can get into trouble.
The other thing I do with my DD who is struggling with school, is to ask her when she gets in what the best bit of her day is, worst bit...whatever.