Read your by-laws and follow the procedure for cancelling your partner's shares. If he's 51% it's likely you'll need his consent to cancel the shares or sell them to you for a nominal value. Document everything and make sure you have corporate minutes showing that the company has approved of the action you're taking.
The foregoing is not legal advice nor is it in any manner whatsoever meant to create or impute an attorney/client relationship.
Bob is right. For later protection be sure to document and keep good records. Your corporate ByLaws are the company version of a Constitution. It covers shareholders and how they transfer ownership. Follow it to the letter. It might not hurt to involve a Business Law attorney. See Avvo.Com under Find-A-Lawyer. Good Luck.
See if Chris will sign something to sell his share back for a nominal value - ie $5 bucks since he has no interest. Otherwise its difficult to change the company ownership without his consent.