Should I tell my ex?

Me and my ex dated for a few months but ended things on a mutual note. We remained pretty close after. Lately we’ve both been busy so it’s been a lot less closer. Last week his close friend messaged me on Instagram and we actually get along very well. We met up at a park with our dogs, had fun, and decided to make plans for the next day (since he didn’t work). He actually went out to lunch with my ex that day before we hung out. Eventually he made a move but I stopped him at first. I told him it was a bit weird because Im technically his friends ex and there is rules about this kind of stuff. He asked what my relationship was like with my ex and I told him we were still friends. He admitted it was a bit weird but that he really liked me and wish we met under different circumstances. He offered to continue things in secret which is where we are at now. I said I don’t like being a secret but that I do like him. When he made another move I let him. Now we kiss and go on dates (well have gone on one since). He even asked me to be his date for an event 3 weeks from now. My ex asked to hang out right now and I’m about to see him. It’s going to be the first time I see him since I started talking to his friend. I’m supposed to see his friend tomorrow for another date. I feel a little strange because I know there’s nothing between me and my ex and we are cool with each other. I just don’t know if he’s going to be upset with either of us. I feel like he would just be weirded out but I’m not certain. Should I tell him we are just friends or what we’ve been doing ?What if his friend doesn’t want me to say anything?