I believe that gay and straight belong on a continuum. Some people are on the firm ends of the spectrum - totally straight or totally gay. Bisexuals are at the middle. But there's no reason one can't be mostly on one end but with a bit of curiosity. I see what he's doing as being no different to a straight who identifies as bi-curious.
What do you do? Well that depends on what you want your life to be. Are you Ok being his experiment?
Never regret anything that has happened in your life. It cannot be changed, forgotten or undone. So, take it as a lesson learned and move on.