It seems obvious you two were romantically involved. Living with friends can be bloody difficult — living with former lovers even more so. What is particularly stressful for you in this situation is that, despite having "moved on" from whatever relations in which you two were engaging, your roommate desires a continued social intimacy. But you want to keep him at arm's length.
You have to talk with him. Tell him what you told me — it could really alter his behavior (I'm going to guess he has no idea you feel this way). Living with the tension for the next year is not worth it.