I STRONGLY suggest that your partner/roommate seek medical evaluation. It sounds like he might be suffering from manic depression. There is no need for him to suffer daily with a negative outlook (energy) if medication can bring balance to his life. When a person is saturated with negative energy, their thinking and logic become cluttered, and joy becomes elusive. He needs to clear the clutter, and that is going to require energy work, and possibly medication.
I suggest that you both seek the services of a Reiki Master who can clear the negative energy. Make sure you see a Reiki Master, not someone who is still learning while practicing. In your case, releasing negative energy should become a daily occurrence during his bouts of depression. I strongly suggest "Ethereal Meditation, meditations and visualizations for health, healing and spiritual clarity", which is available on Amazon. It will teach you how to control the energy around you and keep your energy field clear of negative debris. You cannot control his energy, but you can learn to protect yourself from it, and even clear and transmute that energy, which will also help him. Another suggestion is a Native American tradition of saging the home with a white sage stick, which clears negative energy from a space. Sage sticks are easy to find online. There is also a barrier oil by Ethereal Realm that you can purchase on Etsy. If you rub the oil on your throat and solar plexus chakras, it provides a protective barrier that fends off negative energy. It works well. I hope this helps you and good luck.