Not necessarily. Typically your contract will be with the company you co-founded. Your co-founder is entering your company into a contract with you by signing as an authorized officer on behalf of the company. If that contract assigns the IP to your company, your cofounder is stealing your company's IP. If the contract says the source code is your IP (and perhaps licenses its use to the company for some permitted purpose), then your co-founder is stealing from you.

Now, if you can show the sole purpose of him forming the business with you was to steal your source code, then that's fraud.