I'd say, tell him "I'd like to see if we can salvage this relationship together. What I'm going to need, if we do it, is new level for us of trust, transparency, and a willingness to give and receive communication fearlessly." (Practice saying this to a mirror.) See if he follows through. If not, move on.
> ...who I thought was open to us pursuing an
> open relationship at some point as he is much older
> than I am.
WTF? How does A imply B here? What on Earth was your assumption? Examine what else you may be falsely assuming, and why.