For that objective, you need to make sure he is near and/or you're locked onto him. Not locked onto him as in having to press LT, but more to the point he is simply highlighted.
If you've played any previous AC games, there's a safe way to get off buildings when locked onto any NPC. While looking over the edge, you look at them and press B. You character will then jump down, and use them as a "pillow" to halt you from landing on the ground and hurting yourself.
It was a kind of way to get down off a building without having to climb down or assassinate someone. So with this task, you're doing the same. I did it first time, and all I did was just hold down RT, and I jumped on the little awning of the steps, and the NPC was just a little ahead of me. Once I was on top of it, the NPC was already highlighted, so I just pressed B.
Connor then jumped a nice little distance and all he did was jump onto him. May take a few attempts, but that's how to do it.