Can my boss require I have a letter signed releasing him of liability from side work?

I currently am employed in charlotte, nc. I am having issues with my employer, its a small business & he is very controlling. We are on salary; we work way more than 40 hours a week and have to use our own personal cell phones. When i was hired in my contract it states that i will be eligible for the quarterly bonus program, well there is no bonus program & we never get any bonuses. Since my field is where I have a degree & training I decided to obtain side work. My boss learns about my side work & is requiring me to obtain a written letter from my clients stating that his company has no liability for my work. I never signed any agreement or had a discussion with him about side work. I would never take business away from him, but after asking for my side work to go to his business and he would pay me accordingly and me saying no, he now wants the liability release. I have my own liability policy. I am nervous & uneasy about working there. What can I do?