I'm not a doctor but I from my experience - do NOT let this go unconfronted. I would also carefully plan how to bring it up and possibly talk with a professional first. Now, based on the information you have given I would highly suspect it was more innocent than you think but... if he's had a sorted past and if he has been diagnosed with Bipolar or Borderline, this could be an identity issue that he's been dealing with all his life.

Or he simply could bisexual and there's nothing wrong with that. If you want to talk more in depth, PM me. I have a lot of in sight into this.