if you are asking for poll flexion it is always best to have the horse's jaw slightly flexed to one side or the other . Not only will you horse almost always be "positioned" to one side or the other in dressage, this lateral flexion helps to break up a horse's resistance to the bit when asking for poll flexion.
A horse that has been resistant to, or leaned upon, contact for a long time will have big muslces on each side of the upper neck, where he has a habit of bracing against the bit. By having the horse learn to tip it's jaw to one side, ever so slightly, while flexing at the poll, you make one the horse less likely to lean against the contact with both sides of his neck. And once you have a bit of a "break" in his resistance, you might even be able to talk him into softening on his own. Your horse may not have that habit, but in any case the approach is the same. That slight softness in the jaw to the inside will also help to encourage softness in the body, since if the jaw is braced, the hrose is braced throughout the body.
Also, flexions at a stand still are not really as helpful as flexing the horse while he is walking. If you can get him to flex at the poll, while walking forward, then while maintaingng the soft contact, ask him to carry that forward into a long and low, he will start feeling more comfortable with being connected to the bit. I mean, if you can allow him to both flex, then go long and low, then come back up and be on contact, then down, he will feel more comfortable, knowing that he will not be held in a tight frame. This is stretching/compressing in a longitudinal balance. And for a horse that might not trust your hands with a firmer contact, this might help him .