You're right. So let's just get this straight - when you're lunging your horse (roundpen or otherwise), he's working long and low, tracking up, with his poll at his withers or lower? If not, then he needs more groundwork.
If yes, then I'll give you the other perspective. If the horse is going perfectly on the ground and has a beautiful topline and can move wonderfully with a rounded back, then the issue undersaddle is 100% the rider (assuming it's not pain related). If he already knows how to move correctly, you're not asking him or allowing him to do this undersaddle. And in that case, a german martingale still won't fix your issue because it's not going to fix the rider. You need to get him moving out, forward forward forward with lots of suppleness and transition exercises as others have already stated. And honestly, a video would be a huge help here so we can see what the problem really is. You gave a pretty vague first post - and we gave you the recommendations to fix it that would, going by what you said, help in most scenarios. Getting defensive helps nobody.