A couple months ago this would happen to me also. I think this happens when you are too far away. On the ramp itself it starts out as a small incline then comes to a flat area then declines down to the boss area. If i remember correct if I parked my toons on the small incline area I would have him despawn/reset. IF you move up a little bit to the flat part of the ramp then he should drop the rider and fly around while shooting fireballs at you.

I was frustrated with this for a while and skipped him. Once I learned the placement it was easy.