If the document you signed for the design development phase of the project states clearly that the architect (designer) will provide both schematic and design development plans (drawings) and does not pre-condition that provision with the signing of yet another document, then the designer is legally bound to provide copies of those drawings. He is not bound to give you the originals. If he's balking at living up to his end of the agreement, I'd do two things. First, I'd contact my lawyer just so the legal wheels commenced to turning. And two, I'd let the architect know that the signed agreement requires him to provide you with the plans, and if he still balks at that, I'd let him know that your lawyer has been appraised of the situation.
One more thing...I'd find another designer to work with from this point forth. This issue shouldn't be what it is, and this 'designer' you have is overstepping his bounds. He has, by that action, established quite clearly what his character is and what your future relationship with him may very well be. 'Extortion' is a word that comes to mind here quite clearly.