Start with setting up a treat cup inside the cage. Any time you walk by the cage slowly, drop a treat into that cup. Once he gets more comfortable with you and looks forward to your approach, you can then start working on target training *through* the cage bars. Once he'll reliably target to any location, you can then move on to target training through the open cage door and around the outside of the cage before even attempting to target train away from the cage.
Station training may also come in handy.