he sounds like my dad's old boy. He sucks back behind the vertical and you can throw the rein at him and he'll just sit in that position. If your horse is sucking back like that then holding the position when you drop the reins, he has not accepted a contact. I have two main things that seem to work with our old boy. He is 21 and has apparently dropped behind the vertical for his whole ridden life. The first is to warm him up on a long rein with lots of leg, and once he is waling briskly forward with a stretched neck, I'll start taking up some contact while keeping that marching pace, and everytime he drops back, I'll throw my reins at him and kick him forward again.
What I finds works best however, is shoulder in. I warm him up like usual, then put him into shoulder in in trot for as long as it takes. So basically I'll ride half the long side, then we he takes up the contact I'll ride a 1/2 20m circle, then shoulde rin again etc. It gets them active behind and encourages them to take up a contact.