I would suggest working on this in hand, rather than longing.
Do side flexions and lateral work at the walk, in hand. Get him bent in the direction of travel, rather than counterbent. Get the correct muscles activated. Then you can proceed to doing this at the trot, as long as you have the stamina! It is much easier to teach giving to the bit and stretching to contact when you have the actual bit in your hands. Once he has the idea, you can start longing him on progressively larger circles, pushing his haunches out.