No, though the assignment and partial assignment of copyrights can get complicated and can rely heavily on what is specifically agreed upon in the contractual language. For instance, just based on logic and reason, he cannot state that though that he entered into the agreement with cannot access and use this copyrighted material and claim that they are infringing... no matter what the contractual language states. He cannot unilaterally revoke this contractual agreement without being in breach, and thus owing damages for the breach.
He may, depending on the language, be able to control whom the 50% partner allows to use the work...however, if there is no limiting language in the contract, he may not be able to take that approach either.
So, your instincts appear to be correct. Moreover, it sounds as if this guy has either breached or is attempting to breach the very contract to which he signed, which can result in an award of damages for breach of contract.
By the way, I am a licensed IP attorney with over a dozen years of experience handling matters such as this one.
Please rate my answer positively and let me know if you have any other questions.