Many people work second jobs, including freelance positions. But your other business should stay your business.
Companies often develop and expand non-competition agreements and policies instructing employees to retain only one job. As your company could implement such a policy at any time, you should avoid telling your boss about other work obligations so you can continue to fly under the radar and make ends meet.
Not to mention, if you have an annual review with your boss and he/she cites a decrease in your performance, your boss could easily point the finger at the time and energy you're spending working at the second job. Don't give anyone any excuses to question your work ethic.