Each contract will specify who owns what if you are independent. If the company owns everything then they usually are paying by the hour and IP is a work for hire. If you are delivering consulting or a product for a fixed price then it can be negotiable. For 20 years I have had a mix of both types of contracts.
He also doesn't have to volunteer everything an employee thinks. I know of several employees who came up with ideas while employed and went out and started their own business around that idea. However, you are always open to being sued if the company thinks it has a good case that you developed that idea while doing work for hire.
Either way most reputable companies spell this out specifically as to who owns what.