What do I tell him

Five months ago I left my fiance after being unhappy in our relationship for a long time. It was clear to me that even though he proposed, he was not ready to get married. He spent more time sleeping than anything else and struggled with his mental health through anxiety and depression. For seven years I stayed strong and supportive, but the past year and a half of our relationship he stopped trying to manage his mental health and let it consume him. Our relationship became less if a relationship and more about me taking care of him when he gave me the time of day. He wouldn't call, or text and if I went to his house he would send me home. He thought we were fine

In the last month of the relationship I realized I had feelings for my best friend of 9 years. Who I am now in a happy much healthier relationship with. Yesterday I went to the mall and ran into my ex, who ran and hid in the stock room of his job, before texting my best girlfriend. She asked if I would ever be friends with him, should he move on...which I guess he hasnt. Friendship was what we agreed to when we split. Which turned into him avoiding me. Last night he texted me, just asking how I've been, but I know he has a plan for the conversation...idk what to do. I'm certain he isn't ready to talk to me. And idk if I'm ready to talk to him based on the anxiety o am having thinking about what to say. How did you guys handle a situation where your ex messages you in hopes of moving on and building a Friendship?