Still waiting for that professional . . . But in the meantime, I agree with Rookie in that you reward him rounding his back. When he understnads that that is where he gets a release, he will be more willing to put out the effort, and it does take more effort, to do this .
I like to imagine that I ask the horse to stretch forward, I open the reins out to the side a little bit , a bit like driving a wheelbarrow, keep my upper body from falling forward , and put some leg on and kind of push him "through" the V that you have formed with the reins to his bit. Think of pushing him though it but "up and over it" as if he were going over a small jump , like low hoop that you put out in front of him and to get over it, he must push from behind.
Doing long land low WILL put more weight on the forehand, but with the horse stretching his neck, lifting it from the base and arching it down and forward, and actively driving from behind, even thought the forelegs take on more weight (due to forward position of head) the arching "drive" will lift he back and string the horse from leg to bit, connecting the hand to the hindquarters, and is still a valuable training activity.