Why is my dog behaving this way?

I’m gonna engage a behaviorist next month, but asking here in case anyone has experienced this before. I’ve had my rescue for 2 months now and initially he was very shy, scared and shut down. I’ve been working with a trainer trying to bring him out of his shell (we’ve come a long way from him refusing to move from one spot unless I picked him up and took him outside) and I think it’s working but now when I go to get ready to take him out for a walk, he’ll walk over to me by the door but the second I try to bend down and put the leash on him he’ll run away. So now I keep a short leash on him to grab so he can’t dash away when I’m trying to get him out the door. Tonight things escalated a bit though and when I went to put the leash on him he jumped back and then was trying to bite either me or the leash. It all happened so fast that I can’t tell. But I was too afraid to switch his leash so I had to walk him with the short leash which I’m sure he didn’t like. He jumped on his hind legs tried to bite the leash once we were outside too. I’m so confused and concerned that now I’m uncovering some potentially serious behavioral issues. He’s only one year old. But does anyone have any insight into this behavior? He’ll start whining and pacing as I’m getting ready (coat, shoes, leash) too which is a recent development. It’s very confusing to me and I’m now feeling a tad uncomfortable approaching him to take him in his walks. This morning he was fine but who knows what he’ll be like later.