As a lawyer, I can't give advice over a forum such as this for your specific case but I would suggest two things:

1. This one is complex enough that you should see a competent lawyer in your jurisdiction to confirm technical points such as who owns the code, whether an oral partnership exists (see, e.g., my comments on this at http://news.ycombinator.com/item?id=1645130 for an analysis of this sort of issue under California law), whether you would or would not have a right to use your code for other work without obligation to your former co-founder, and (above all, given what your co-founder is asking) whether your co-founder has any legal leverage to force you to sign the "nda" or to do whatever else he is trying to force you to do.

2. Most important, I consider what I would advise a hypothetical person in the position of your co-founder as an abstract proposition: you don't have possession of the code and did not develop it or pay value to have it developed on your behalf; you have no work-for-hire agreement giving you rights to the IP, meaning that in your best case you may have only an implied license to use it and nothing more; you came up with the idea but most ideas are not protectable as such unless they are patentable; you might have a claim against your departing co-founder on a theory that you disclosed a trade secret to him in confidence as part of a proposed alignment that never occurred and, on that ground, might be able to claim that he misappropriated a valuable trade secret from you if that secret is truly proprietary and gives you a significant competitive advantage; the foregoing will not apply if the idea is of a type that is being pursued by others, as this puts it in the public domain and vitiates its trade-secret status; you may be held to have waived confidentiality by disclosing this trade secret to your erstwhile partner without having him sign a confidentiality agreement in connection with the disclosure; to enforce this claim, you would need to spend potentially thousands of dollars in a lawsuit fighting over it; in the meantime, your ability to do anything with your idea would be severely impaired because you would have to explain to anyone who wanted to join your early-stage company that you would be devoting major portions of your limited startup resources to an expensive legal fight or, alternatively, that a cloud hangs over your venture in the form of a possible major legal claim by your former co-founder; this sort of thing is virtual death for most startups and serves to dissuade both prospective co-founders and investors from becoming involved with your company; and, last but not least, unless your erstwhile co-founder capitulates and signs documents giving you a clear path to go forward, he can simply sit and wait and potentially assert partnership and other claims against your company at any time it builds significant value, subject only to the running of the applicable statutes of limitations and to certain equitable doctrines that don't him to lay back forever in hopes of potentially sandbagging you. Likely advice to him: "Therefore, threaten away - your main leverage at this point is simply to intimidate."

Given the above, I agree with the many good comments in this thread that it is probably not wise to get embroiled in a legal fight over this situation - yet, this doesn't mean you shouldn't consider using use the leverage you do have to negotiate, if possible, a mutual walk-away that gives you a workable path to use your code in a separate venture entirely, as for example by signing documents that give each party the right to pursue this independently going forward without obligation to the other party. This may or may not be a good option but you should discuss it with your lawyer to see if it might be effective here.