We were in this very same situation. Like yours, my son is developmentally delayed but he also has some aspergers tendencies. He is 5 1/2 now. I know this isn't what you want to hear, but you honestly have to wait him out. When he is ready he'll let you know.
We tried everything. I mean everything. A friend even recommended we teach him first to pee on a tree because he might find that funny and it didn't work. He liked his diapers!
We had given up, when one day out of the blue he announced he had to go potty and went. Just like that. And he had no more problems with it.
We couldn't get him to sleep without a diaper until about 3 months ago, but waiting for him to be reading was the best decision. Every morning he wakes up he's so proud, he can't wait to tell me he didn't pee the bed.
at 11:45 PM on Oct. 2, 2010