My son won't toilet train?

He is nearly 3 and a half and he has no interest in using the toilet. He knows you have to do a wee and a poo in the toilet and he watched my husband and I using the toilet. I've told him that as he is a big boy, big boys use the toilet and wear pants. He likes wearing pants (I won't put him in pull ups because I think its the same as a nappy) and he will sit on the toilet but he never actually goes. We read books, sings and I've even resorted to bribery by letting him have some sweets! Sometimes, after sitting on the toilet, he will have a wee 5-10 minutes later in his pants and then shouts that he's had a wee and wants to be changed. Sometimes he will take off his pants and trousers himself before I even get to him. He clearly doesn't like the fact that he is wet and I have tried leaving him in his wet pants and trousers so he understands that its not nice being wet but that doesn't work either. I don't tell him off, I just ask him what he should have said and he replies "mama, toilet, quick". After the accident in his pants, he does ask to go to the toilet but, again, nothing comes out (it wouldn't if he's already been in his pants!). I am getting concerned now as he starts nursery at school this September and I know he won't be taken if he's still wearing nappies. I don't know if the fact that he was 10 weeks premature has anything to do with it. He is very bright in all other areas. I mean, even with toilet training he knows what he has to do, he just wont do it! I'm getting desperate now so any help and advice would be greatly appreciated.