I believe that was George Noory. If you recall, Art tried to retire once, but couldn't, because he hadn't properly grooved in a competent replacement.
From what I remember, he'd had George fill in for him several times already, so George took over full time during Art's first abortive attempt to leave. I think I'm recalling that episode correctly, but it was about ten years ago, so my memory of it may be off a bit.
I distinctly remember when George officially took over after Art's final departure. The man was so fake and wooden, I could hardly stand to listen to him. He got better (and more relaxed) over the years, but he's never matched Art's level of skill and ability behind the mic.
I always noticed that too; he could have some kooky people on, but always treated them with dignity and respect; even in the face of incredulity.