I'm not going to tell my ex that I was hooking up with again that I slept with someone else
Why do you want to keep a secret like that?
I don't think they should know about the other person.
Even though your relationship sounds confusing, you probably should tell your ex about what you did if you want to keep on working things out with him. Lying to a sexual partner is wrong because they might not be as careful as they should be if they thought they were the only person you were sleeping with.
I feel like telling him is a waste. I thought we had something going on.
I think that part of being in an open relationship like that is being honest. When you are sleeping with someone honesty about things like this is going to be the best policy.