People always say "go talk to a lawer!" but someone in this situation likely has no idea how to even find the proper lawyer.
There's a misconception that meeting with a layer means you 1) have to hire them and 2) that you're going to file a lawsuit. Neither is true. Most good lawyers offer a free consultation and you're just seeking advice, not a litigator.
OP: the code and intellectual property belongs to you unless you transfer copyright over or mark as open source. That said, code is hardly the most valuable asset in an early-stage startup. The best real way to "protect" yourself (besides getting legal docs papered) is to stay involved in every important discussion and develop your own relationships with investors. Your cofounder should welcome this, and should help you develop your non-technical skills.
If you notice any unease, red flags, shadiness, lack of transparency, BAIL OUT. Incorporate yourself and transfer the IP into a company owned 100% by you. Find a new business partner.
The OP said he's working for another company while he's doing this ("I have a very well compensated job in a local startup"). Depending on the employment agreement he signed with that company, they may have rights to the code he wrote (especially if he wrote any part of it using the company's equipment or on the company's premises, or if the code is related to the company's line of business). Remember, this is in NY, not in California, so the protections against unreasonable employment contracts could be weaker. So he really needs to talk to a lawyer before taking any kind of action.