If you didn't sign any documentation you should be in the clear. However I would definitely create documentation about when you work on it, that you weren't usi

If you didn't sign any documentation you should be in the clear. However I would definitely create documentation about when you work on it, that you weren't using company equipment (computer, reimbursed internet, phone etc.) in the process of creating the project. Until the project is up and running I probably wouldn't inform him in case you risk losing one or the other.