I would create a new legal entity to own the IP and divide the shares as seems appropriate to the founders. The new entity can enter into a license agreement (exclusive or otherwise) with the pre-existing business.
Also, there's a risk that your partner (or others with influence in his company) holds you over a barrel at some key point. Is your partner a lawyer giving legal advice for the whole Newco? If so, you may need to consult, even if just informally, another equally seasoned professional who can consider your interests and the Newco's interests without the mixed loyalties inherent in your partner and his company.
(If I sound wary, it's because I've seen a business partnership between one legally-savvy friend and another trusting friend degenerate into acrimony and lawsuits.)