Uni flatmates

Okay, to cut a long story short I recently sorted out my uni accomodation for next year, however I now cannot stand my current flat mate who was meant to be my flat mate next year. Due to an incident that occurred that left me uncomfortable and really distressed, his lack of care over a situation he caused has caused me to rethink my living arrangements next year. I have been avoiding him for days and he doesn't seem to think he's done anything wrong. How do I deal with this situation? Do I be upfront, tell him I'm mad and no longer going to live with him next year? Or do I bide my time as I only have a few weeks left at uni and avoid him, and later tell him via message that I will not be living with him? I find it really hard to deal with confrontation and this whole situation has been giving me a lot of anxiety.