Your partnership agreement will control how the partnership can be dissolved. There should be a buyout provision or a dissolution provision. I suggest that you speak to your partner first to see what his/her intentions are and then move forward from there. In your question, you start out stating that it is a partnership but then discuss his opening a second business under the corporate name. Not sure, if it is a partnership or a corporate entity. Makes a difference regarding getting out of the business.
My comments are not intended to establish an attorney-client relationship, are not confidential, and are not intended to constitute legal advice. Proper legal advice can only be given by an attorney who agrees to represent you, who reviews the facts of your specific case, who does not have a conflict of interest preventing the representation, and who is licensed as an attorney in the state where the law applies. less
Assuming you are a corporation, and not an unincorporated, unregistered informal partnership, you should normally have bylaws and articles of incorporation (organization) which deal with how you wrap up the business. Realistically, you should contact an attorney, and provide details such as whether there are any organizational documents (articles of incorp, bylaws, shareholder's agreements, etcetera), whether there is business debt outstanding, whether you've given any personal guarantees for business borrowings, including equipment or office leases, whether the partner's second business competes with the existing business, whether there are any employment agreements specifying what the partners are supposed to be doing for the original business, etcetera. Arguably, as an operation of the existing business, profits from the second business should be available to all shareholders of the corporation.