Most Helpful Guy
Im sorry to hear this. I think you should have an honest conversation with him, and let him know where u stand. As your partner if he really cares he would take what u said into account and act on what he feels and thinks. If what he says and does doesn't align with u. Would u stand and tolerate it. If so then that's how it's going to be and u have to be ok with it. If not then you really have to consider if u want to be with him for the rest of your life.
Most Helpful Girl
Before you cheat. What I making him drift away? Is it his games? Or is he depressed? Discuss on whats going on before you take action. It will be fun , cheating... but you will feel guilty and wished to fix it before making it worsen.