Should I be open with my ex fiancé?

So my fiancé and I broke up a few months ago but we both are still in love and he said he is not giving up on us. We are not together right now but an old friend of mine who is the cousin of one of my best friends wants to hang out. We are going to workout and watch a movie at his place. Now I don't want a romantic relationship with him we literally are just cool and I've known him since I was 18 and I'm now 28. I know that I am single and can do what I want but if my ex fiancé was to find out he will flip out. So I feel if I am open with him and tell him up front that he will not be so upset. Trust me nothing is going on with me and my guy friend and I still love and want to be with my ex so I think I just owe him that open honesty. What are your thoughts?

In case y'all are wondering why we broke up well a lot of things happened and we just decided that our relationship was turning toxic and we needed to breakup but we are very much still in love. I am a very open person and I don't like to be sneaky or hide things especially when nothing is going on you know?