Please help, my son hates being without a nappy!!

My son is 3 in June, we had a few dry days in NOv and I thought we had cracked it, but then he started crying if I left his nappy off, now he wont go without it, he knows when he needs to go as if he's in the bath he will tell me he needs a poo and will get out and go on the toilet, said he doesnt want to use the potty or big toilet, have let him choose his own pants and potty but still wont use them?? help, he;s due to go up to pre school in nursery next month but they have said they wont take him if hes not dry???
I dont know what to do and feel like a useless Mummy??
Marie..xx