Horses: Improving the free walk and creating more elevation at canter...?

My horse is really well educated all ready, he's done a ton of WP and W. trail classes, and for the last year and a half I've been showing him hunt seat. We also jump up to about 2' (that's about his confidence limit right now). He is a 9 yo APHA gelding.

So this winter I want to get back into dressage with a new friend at the barn. My horse has all the lateral movements all ready.... side passes, half pass, turn on haunches, turn on fore etc. He is excellent as long as he's on the bit.

Where I have trouble is the free walk. Remembering that he was originally a WP horse, his walk is like a 0 in dressage. If you relax and encourage him to stretch down into the bit, he sticks his head way down into WP frame. Grossly down. And his pace is grossly slow. If he's bridled up I can get a lovely medium walk, collected walk, and an extended walk. But the minute I drop contact his head goes down into WP land. WAY WAY DOWN. Then using leg to encourage movement, I get a WP jog. I've tried breathing my legs behind the girth a bit and I've tried alternating leg pressure, both with the extending rear leg and with the opposite side. I've tried everything.

What exercise can I do to break through the WP trained walk? I've also tried having him in a very extended walk and slowly dropping contact while keeping the impulsion with my legs, but his head goes very low and I get the WP jog....