He can only do that if he has clear legal title to it --- and the issue here is precisely that he's signed a contract which may give the employer first dibs to

He can only do that if he has clear legal title to it --- and the issue here is precisely that he's signed a contract which may give the employer first dibs to appropriate it. (Beyond that, things get into murky "consult a lawyer" territory very quickly --- the precise language of the contract may make a real difference. And so may questions of jurisdiction; California law is more employee-friendly than elsewhere in the US, but you can't necessarily take that to the bank without checking the contract and the circumstances. Things get a whole lot easier if you can get the employer to sign papers which formally waive their claim on whatever-it-is --- but ideally, you'd consult a lawyer on the wording of the waiver. I've got one for my own open-source side projects, but it hasn't been legally vetted for third-party use, so I'm a bit reluctant to share it...)